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Q1 What are the different data types present in javascript?

n JavaScript, there are several data types, including:

1. Number: represents numeric values, including integers and floating-point numbers.
2. String: represents text values, enclosed in single or double quotes.
3. Boolean: represents a logical value, either true or false.
4. Null: represents an intentional absence of any object value.
5. Undefined: represents a declared variable with no assigned value or an undeclared variable.
6. Object: represents a collection of properties and values, or a reference to a function or array.
7. Symbol: represents a unique value that is not equal to any other value.

In addition to these primitive data types, JavaScript also has two special data types:

1. NaN: represents a value that is “not a number”.
2. Infinity: represents a value that is greater than any other number.

It’s important to note that in JavaScript, variables do not have data types, only values do. This means that a variable can hold any type of value at any time, and the data type of the value will determine how the variable can be used.

Q2 Explain Hoisting in javascript.

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their respective scopes during the compilation phase, which happens before the code is executed.

This means that variables and functions can be used in code before they are actually declared, without causing a ReferenceError. However, only the declaration is moved to the top, not the assignment or initialization of a variable.

For example, consider the following code:

console.log(message); // output: undefined
var message = "Hello, world!";

Even though the variable message is declared after it is used, the code still runs without error because the variable declaration is hoisted to the top of the current scope. However, since the variable is not initialized until later in the code, its value is undefined at the time of the console.log() call.

The same is true for function declarations:

sayHello(); // output: "Hello, world!"
function sayHello() {
  console.log("Hello, world!");
}

Here, the function sayHello() is declared after it is called, but it still runs without error because the function declaration is hoisted to the top of the current scope.

However, it’s important to note that hoisting can lead to unexpected behavior if you’re not aware of how it works. It’s generally considered best practice to declare variables and functions at the top of their respective scopes, even if they’re not going to be used until later in the code. This can help avoid confusion and bugs caused by hoisting.

Q3  Why do we use the word “debugger” in javascript?

The term “debugger” in JavaScript refers to a tool or software that helps developers identify and fix errors or bugs in their code. The name “debugger” is derived from the term “debugging,” which refers to the process of finding and fixing errors or bugs in software.

The term “debugging” was first used in the context of computer software in the mid-20th century. At that time, computer hardware engineers used a device called a “bug” to locate defects in circuits. If a circuit was not functioning correctly, the engineers would use the “bug” to find the source of the problem, which was often a small physical defect in the circuit board.

In the early days of programming, errors in code were often caused by physical defects in the hardware, such as faulty memory chips. As software development became more complex, however, errors began to arise from mistakes made by programmers. To locate these errors, programmers began using tools that allowed them to step through their code one line at a time, and to inspect the state of their program at each step. These tools became known as “debuggers” in honor of the earlier use of the term.

Today, JavaScript debuggers are an essential tool for developers, allowing them to identify and fix errors in their code quickly and efficiently. They can be built into the browser or included in integrated development environments (IDEs) and code editors.

Q4. Difference between “ == “ and “ === “ operators.

In JavaScript, the “==” and “===” operators are used to compare values, but they do so in slightly different ways.

The “==” operator performs type coercion, meaning it converts the operands to a common type before comparison. For example, the expression "5" == 5 will return true because the string “5” is converted to the number 5 before the comparison. This can lead to unexpected results if you’re not careful.

On the other hand, the “===” operator does not perform type coercion, and it only returns true if the operands are of the same type and have the same value. For example, the expression "5" === 5 will return false because the operands are of different types.

Here’s a quick summary of the differences:

• “==” performs type coercion, while “===” does not.
• “==” returns true if the operands have the same value after type coercion, while “===” only returns true if the operands have the same value and type.
• “===” is generally considered safer and more predictable, while “==” can lead to unexpected results if you’re not careful.

It’s important to understand the differences between these operators and use them appropriately in your code to avoid bugs and unexpected behavior.

Q5. Difference between var and let keyword in javascript.

In JavaScript, var and let are both used for declaring variables, but they have some key differences.

1. Scope: var has function scope, while let has block scope. This means that a variable declared with var is accessible within the function it is declared in, while a variable declared with let is only accessible within the block it is declared in (e.g. inside a loop or conditional statement).
2. Hoisting: Variables declared with var are hoisted to the top of their scope, meaning they can be accessed before they are declared. Variables declared with let are not hoisted, and if you try to access them before they are declared, you will get a reference error.
3. Redeclaration: Variables declared with var can be redeclared within the same scope, which can lead to unintended consequences and bugs. Variables declared with let cannot be redeclared within the same block, which can help prevent these kinds of errors.

Here’s an example that demonstrates these differences:

function example() {
  var x = 1;
  let y = 2;

  if (true) {
    var x = 3; // redeclares x
    let y = 4; // does not redeclare y

    console.log(x); // output: 3
    console.log(y); // output: 4
  }

  console.log(x); // output: 3
  console.log(y); // output: 2
}

example();

In this example, x is declared with var and is accessible within the entire function. When it is redeclared within the conditional statement, it affects the value of x outside of the block as well. y, on the other hand, is declared with let and is only accessible within the block it is declared in. When it is redeclared within the block, it does not affect the value of y outside of the block.

In general, it is recommended to use let instead of var for declaring variables in modern JavaScript, as it helps prevent unintended consequences and bugs.

Q6. Explain Implicit Type Coercion in javascript.

Implicit type coercion in JavaScript is the automatic conversion of one data type to another by the JavaScript engine, without the need for explicit conversion code. This happens when a value of one type is used in a context that expects a different type.

For example, when we use the “+” operator with one string and one number, JavaScript will implicitly convert the number to a string and concatenate the two values:

let x = 5;
let y = "10";
let z = x + y; // z = "510"

In this example, the variable x is a number, and the variable y is a string. When we use the “+” operator to combine them, JavaScript will implicitly convert x to a string and concatenate it with y.

Implicit type coercion can also happen when comparing values with different data types

let a = 5;
let b = "5";

if (a == b) {
  console.log("Equal");
}

In this example, a is a number, and b is a string. When we use the “==” operator to compare them, JavaScript will implicitly convert b to a number and compare it with a. In this case, the values are equal, so the code will output “Equal”.

Implicit type coercion can be useful in some cases, as it allows for more concise and readable code. However, it can also lead to unexpected results if you’re not careful, so it’s important to be aware of how it works and use it judiciously. To avoid unexpected behavior, it’s often a good idea to use explicit type conversion code (such as Number() or String()) when you need to convert values between different types.

Q7. Is javascript a statically typed or a dynamically typed language?

JavaScript is a dynamically typed language. This means that the data type of a variable is determined at runtime, not at compile time. When you declare a variable in JavaScript, you don’t need to specify its data type explicitly; you can simply assign a value to it, and JavaScript will figure out the data type based on the value.

For example:

let x = 5; // x is a number
let y = "Hello"; // y is a string
let z = true; // z is a boolean

In this code, the data type of x, y, and z is determined based on the values that are assigned to them.

This is different from statically typed languages, where you need to declare the data type of a variable explicitly, and the compiler will check to make sure that the variable is only assigned values of that type. In JavaScript, since the data type is determined at runtime, it is possible to assign values of different types to the same variable during the execution of the program.

While dynamic typing can make development faster and more flexible, it can also make it easier to introduce errors into your code, since type mismatches may not be caught until runtime. To mitigate this risk, JavaScript has introduced features like TypeScript, which adds static typing to the language and provides additional type checking at compile time.

Q8. What is NaN property in JavaScript?

NaN stands for “Not a Number” in JavaScript. It is a property of the global object that represents a value that is not a legal number. It is returned when a mathematical operation or function fails to return a valid number.

For example, dividing a number by zero or attempting to perform a mathematical operation on a non-numeric value will result in NaN:

let x = 5 / 0; // x is Infinity
let y = "Hello" * 2; // y is NaN

In the above code, dividing 5 by 0 results in Infinity, which is a special numeric value that represents positive infinity. However, attempting to multiply a string by a number is an invalid operation, and results in NaN.

One important thing to note is that NaN is not equal to any value, including itself. This can lead to some unexpected behavior if you’re not careful:

console.log(NaN == NaN); // false

In this example, even though both values are NaN, the comparison returns false because NaN is not equal to itself.

To check for NaN, you can use the isNaN() function, which returns true if the value is NaN:

console.log(isNaN(NaN)); // true
console.log(isNaN(5)); // false
console.log(isNaN("Hello")); // true

In the above code, the isNaN() function returns true for NaN and for non-numeric values like “Hello”, but returns false for a valid number like 5.

Q9. Explain passed by value and passed by reference.

In programming languages, when you pass a value to a function, it can be passed by value or by reference. This determines how the value is handled and modified within the function.

Passing by value means that a copy of the value is passed to the function, and any modifications made to the value inside the function do not affect the original value outside the function. This is typically used for primitive data types like numbers, strings, and booleans.

For example:

let x = 5;

function incrementValue(value) {
  value = value + 1;
}

incrementValue(x);

console.log(x); // Output: 5

In this example, x is a number and is passed to the incrementValue() function. However, since numbers are passed by value, a copy of the value is passed to the function, and any modifications made to value inside the function do not affect x outside the function.

Passing by reference means that a reference to the original value is passed to the function, and any modifications made to the value inside the function also affect the original value outside the function. This is typically used for complex data types like objects and arrays.

For example:

let myObj = { value: 5 };

function incrementValue(obj) {
  obj.value = obj.value + 1;
}

incrementValue(myObj);

console.log(myObj.value); // Output: 6

In this example, myObj is an object and is passed to the incrementValue() function. Since objects are passed by reference, a reference to the original object is passed to the function, and any modifications made to obj inside the function also affect myObj outside the function.

Understanding whether a value is passed by value or by reference is important for writing correct and efficient code, as it determines how the value is handled and modified within the function.

10. What is an Immediately Invoked Function in JavaScript?

An Immediately Invoked Function (IIFE) is a JavaScript function that is executed as soon as it is defined. It is also known as a self-executing anonymous function, because it is declared anonymously and immediately invoked.

Here is an example of an IIFE:

(function () {
  // code to be executed immediately
})();

In this example, the function is declared and immediately invoked using parentheses. The code inside the function is executed immediately when the page loads.

IIFEs are often used to create a new scope for variables and functions, so that they do not interfere with variables and functions in the global scope. This is known as encapsulation.

For example:

(function () {
  let message = "Hello";
  
  function showMessage() {
    console.log(message);
  }
  
  showMessage();
})();

In this example, the message variable and showMessage() function are encapsulated inside the IIFE, so they are not accessible from the global scope. This prevents them from interfering with other variables and functions in the global scope, and helps to avoid naming conflicts and other issues.

IIFEs can also be used to create private variables and functions, which are not accessible from the outside. This can be useful for creating modular and reusable code.

Q11. What do you mean by strict mode in javascript and characteristics of javascript strict-mode?

Strict mode is a feature in JavaScript that allows you to place your code in a strict operating context. When strict mode is enabled, certain JavaScript behaviors are disallowed or flagged as errors, making it easier to write secure and maintainable code.

To enable strict mode in JavaScript, you simply add the following line to the top of your JavaScript file or function:

'use strict';

Once strict mode is enabled, the following behaviors are changed:

1. Disallows undeclared variables: In strict mode, if you attempt to use a variable that has not been declared, a ReferenceError is thrown.
2. Disallows duplicate properties: In strict mode, if you attempt to define multiple properties with the same name in an object literal or class, a SyntaxError is thrown.
3. Disallows the use of reserved words as variable names: In strict mode, if you attempt to use a reserved word (such as let, const, enum, or implements) as a variable name, a SyntaxError is thrown.
4. Changes the behavior of this: In strict mode, the value of this inside a function is undefined if the function is called without an object. In non-strict mode, the value of this defaults to the global object.
5. Disallows with statements: In strict mode, with statements are not allowed, as they can cause confusion and unexpected behavior.
6. Disallows eval and arguments as variable names: In strict mode, you cannot use eval or arguments as variable names.
7. Enables faster code execution: Strict mode code can be optimized more effectively by modern JavaScript engines, leading to faster code execution.

Using strict mode can help you catch and prevent certain types of errors, and can make your code more secure and maintainable. However, it is important to note that strict mode is not a substitute for good programming practices, and it cannot prevent all types of errors.

Q12. Explain Higher Order Functions in javascript.

In JavaScript, a Higher Order Function (HOF) is a function that takes one or more functions as arguments, or returns a function as its result. HOFs are a powerful and flexible feature of the language, and are widely used in functional programming and modern JavaScript development.

Here are some examples of Higher Order Functions in JavaScript:

1. map(): The map() method creates a new array by applying a function to each element of an existing array. The function is passed as an argument to map(). For example:

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(function(num) {
  return num * 2;
});
console.log(doubled); // Output: [2, 4, 6, 8, 10]

2. filter(): The filter() method creates a new array with all elements that pass a certain test. The test is defined by a function that is passed as an argument to filter(). For example:

const numbers = [1, 2, 3, 4, 5];
const even = numbers.filter(function(num) {
  return num % 2 === 0;
});
console.log(even); // Output: [2, 4]

3. reduce(): The reduce() method reduces an array to a single value by applying a function to each element of the array. The function is passed as an argument to reduce(). For example:

const numbers = [1, 2, 3, 4, 5];
const sum = numbers.reduce(function(total, num) {
  return total + num;
}, 0);
console.log(sum); // Output: 15

HOFs are a powerful and flexible feature of JavaScript, and are widely used in modern JavaScript development. They allow you to write more concise and reusable code, and can help to make your code more expressive and readable.

Q13. Explain “this” keyword.

In JavaScript, the this keyword refers to the object that is currently executing a function. The value of this depends on how the function is called, and can be different in different contexts.

Here are some common ways that the this keyword is used in JavaScript:

1. As a reference to the global object: If this is used outside of any function or object, it refers to the global object (e.g. window in a browser or global in Node.js).
2. As a reference to the calling object: If a function is called as a method of an object, this refers to the object that the method belongs to. For example:

const person = {
  name: 'Alice',
  sayHello: function() {
    console.log('Hello, my name is ' + this.name);
  }
};
person.sayHello(); // Output: "Hello, my name is Alice"

In this example, the sayHello() method is called as a method of the person object, so this refers to the person object.

3. As a reference to a new object being created: If a function is called with the new keyword, this refers to the new object that is being created. For example:

function Person(name) {
  this.name = name;
}
const alice = new Person('Alice');
console.log(alice.name); // Output: "Alice"

In this example, the Person() constructor function is called with the new keyword, so this refers to the new object that is being created.

4. As a reference to a specific object: You can also use the call() or apply() method to explicitly set the value of this for a function call. For example:

const person1 = { name: 'Alice' };
const person2 = { name: 'Bob' };
function sayHello() {
  console.log('Hello, my name is ' + this.name);
}
sayHello.call(person1); // Output: "Hello, my name is Alice"
sayHello.call(person2); // Output: "Hello, my name is Bob"

In this example, the call() method is used to call the sayHello() function with a specific value of this.

Understanding how the this keyword works is an important part of writing effective JavaScript code. By using this correctly, you can write more reusable and expressive code that works well in a variety of contexts.

Q14. What do you mean by Self Invoking Functions?

Self-invoking functions (also known as Immediately Invoked Function Expressions or IIFEs) are functions that are executed automatically when they are defined, without being explicitly called. They are commonly used to create a new scope for variables and functions, which helps avoid naming conflicts and can improve code organization.

Here’s an example of a self-invoking function:

(function() {
  // code goes here
})();

In this example, the function is defined inside a pair of parentheses, and then followed by another pair of parentheses that immediately invoke the function. The parentheses around the function are necessary because otherwise JavaScript will interpret the function as a function declaration, and will expect a function name before the parentheses.

Self-invoking functions can also take arguments, just like regular functions:

(function(name) {
  console.log('Hello, ' + name);
})('Alice');

In this example, the self-invoking function takes a name argument and logs a greeting to the console.

Self-invoking functions are useful in situations where you want to execute some code immediately, but you don’t want to clutter up the global namespace with variables and functions that are only used locally. By creating a new scope with a self-invoking function, you can define variables and functions without worrying about naming conflicts, and then safely execute the code knowing that it won’t interfere with anything else in your program.

Q15. Explain call(), apply() and, bind() methods.

In JavaScript, call(), apply(), and bind() are methods that allow you to set the this value explicitly when calling a function, and to pass arguments to the function. Here’s how each of these methods works:

1. call(): The call() method calls a function with a specified this value and arguments provided individually. For example:

function sayHello() {
  console.log('Hello, ' + this.name);
}

const person = { name: 'Alice' };
sayHello.call(person); // Output: "Hello, Alice"

In this example, the call() method is used to call the sayHello() function with person as the this value.

2. apply(): The apply() method calls a function with a specified this value and arguments provided as an array. For example:

function sayHello(greeting) {
  console.log(greeting + ', ' + this.name);
}

const person = { name: 'Alice' };
sayHello.apply(person, ['Hello']); // Output: "Hello, Alice"

In this example, the apply() method is used to call the sayHello() function with person as the this value, and the ['Hello'] array as the argument list.

3. bind(): The bind() method returns a new function with the specified this value and arguments, without calling the original function. This can be useful for creating a new function that has a fixed this value, or for creating a partially applied function with some arguments already set. For example:

function sayHello(greeting) {
  console.log(greeting + ', ' + this.name);
}

const person = { name: 'Alice' };
const helloAlice = sayHello.bind(person, 'Hello');
helloAlice(); // Output: "Hello, Alice"

In this example, the bind() method is used to create a new function helloAlice that has person as the this value, and the 'Hello' argument already set. When helloAlice() is called, it logs “Hello, Alice” to the console.

Using call(), apply(), and bind() can be very powerful when working with complex functions and objects in JavaScript. By explicitly setting the this value and passing arguments to a function, you can control how the function behaves in different contexts and reuse it in different parts of your code.

Q16. What is the difference between exec () and test () methods in javascript?

In JavaScript, both the exec() and test() methods are used for pattern matching with regular expressions, but they have some differences:

1. exec() method: The exec() method is a RegExp method that searches a string for a pattern match, and returns an array containing information about the match, or null if there is no match. The array returned by exec() contains the matched substring, as well as any captured groups and their positions in the string. If the regular expression includes the g flag, the exec() method can be called multiple times to find all matches in the string. For example:

const pattern = /hello (\w+)/g;
const str = 'hello world, hello alice';
let result;

while ((result = pattern.exec(str)) !== null) {
  console.log(result[1]); // Output: "world", "alice"
}

In this example, the pattern.exec(str) method is called multiple times to find all occurrences of the word after “hello” in the str string.

2. test() method: The test() method is a RegExp method that tests a string for a pattern match, and returns true or false. Unlike exec(), the test() method does not return any information about the match itself. It simply checks whether the pattern matches the string or not. For example:

const pattern = /hello (\w+)/;
const str = 'hello world';
const result = pattern.test(str); // Output: true

In this example, the pattern.test(str) method is used to test whether the str string matches the hello (\w+) pattern.

So, the main difference between exec() and test() is that exec() returns an array of information about the match, while test() returns a boolean value indicating whether the pattern matches the string or not. Additionally, exec() can be used to find multiple matches if the g flag is set, while test() only checks for a single match.

Q17. What is currying in JavaScript?

Currying is a technique in functional programming where a function with multiple arguments is transformed into a sequence of functions that take a single argument each. The output of each function is passed as an input to the next function in the sequence until all arguments are supplied and the final output is returned.

In JavaScript, currying is achieved through closure and higher-order functions. Here is an example of currying in JavaScript:

function add(a) {
  return function(b) {
    return a + b;
  }
}

const add5 = add(5); // returns a function that adds 5 to its argument

console.log(add5(3)); // Output: 8

In this example, the add() function takes a single argument a, and returns a new function that takes a single argument b, which when called, returns the sum of a and b. The add5 constant is created by calling add(5), which returns a function that adds 5 to its argument. When add5(3) is called, it returns 8 which is the sum of 5 and 3.

Currying can be useful in situations where a function requires multiple arguments, but it is not always possible or practical to supply all arguments at once. By currying the function, it is possible to create new functions with some arguments pre-supplied, making it easier to use the function in different contexts.

Q18. What are some advantages of using External JavaScript?

Using external JavaScript has several advantages:

1. Separation of concerns: Separating JavaScript code into an external file allows for a clear separation of concerns between the content of the web page and its behavior. This makes it easier to maintain and modify the code, as changes made to the JavaScript code will not affect the HTML content.
2. Reusability: External JavaScript files can be reused across multiple web pages, which can save time and effort in development.
3. Caching: When an external JavaScript file is loaded, the browser caches the file, which can speed up subsequent page loads as the file does not need to be reloaded from the server.
4. Faster page load times: By placing JavaScript code in an external file, the HTML content is smaller, resulting in faster page load times.
5. Improved readability: External JavaScript files are easier to read and understand than inline JavaScript code, as they can be formatted and commented in a more structured way.
6. Improved SEO: External JavaScript files can be optimized for search engines by using descriptive filenames and including relevant comments, which can improve the SEO of the web page.

Overall, using external JavaScript can help to improve the organization, maintainability, and performance of web pages.

Q19. Explain Scope and Scope Chain in javascript.

In JavaScript, scope refers to the visibility and accessibility of variables, functions, and objects in different parts of a program. The scope of a variable or function determines where it can be accessed and how long it will be available during program execution.

There are two types of scope in JavaScript:

1. Global scope: Variables and functions declared outside of any function or block have global scope and can be accessed from anywhere in the program.
2. Local scope: Variables and functions declared inside a function have local scope and can only be accessed within the function or its nested functions.

The scope chain is the order in which JavaScript looks for variables and functions when they are accessed. When a variable or function is called, JavaScript first looks for it in the current scope, and if it is not found, it looks in the next outer scope, and so on until it reaches the global scope. This is known as the scope chain.

Here is an example of scope and scope chain in JavaScript:

let x = 10;

function foo() {
  let y = 20;

  function bar() {
    let z = 30;
    console.log(x + y + z); // Output: 60
  }

  bar();
}

foo();

In this example, x has global scope and can be accessed from anywhere in the program. y has local scope and can only be accessed within the foo() function, while z has local scope and can only be accessed within the bar() function. When bar() is called, JavaScript looks for x, y, and z in the local scope first, then in the outer foo() function scope, and finally in the global scope. It finds x in the global scope, y in the foo() function scope, and z in the bar() function scope, and computes the sum of x, y, and z to be 60.

Understanding scope and the scope chain is important in JavaScript for avoiding naming conflicts, managing memory usage, and writing efficient and maintainable code.

Q20 Explain Closures in JavaScript.

In JavaScript, a closure is a function that has access to variables from its outer (enclosing) function, even after the outer function has returned. A closure is created when a function is defined inside another function and the inner function references a variable from the outer function.

Here’s an example:

function outer() {
  const message = "Hello, world!";

  function inner() {
    console.log(message);
  }

  return inner;
}

const innerFunc = outer();
innerFunc(); // output: "Hello, world!"

In this example, outer() returns the inner function inner(), which references the variable message from outer(). When outer() is called, it sets message to “Hello, world!” and returns inner(). innerFunc is then assigned the return value of outer(), which is inner(). When innerFunc() is called, it logs “Hello, world!” to the console, even though message is not defined in the global scope.

This happens because inner() has access to the variable message through a closure. The closure preserves the state of outer() at the time it was called, including the value of message.

Closures are commonly used in JavaScript for creating private variables and functions, as well as for creating callbacks and event handlers. They can also be used to create functions that return other functions with different behaviors based on the closure’s context.

Q21. Mention some advantages of javascript.

JavaScript has several advantages that make it a popular choice for web development:

1. Client-side scripting: JavaScript is a client-side scripting language, which means that it runs directly in the browser of the user. This enables web developers to create interactive web pages and applications without the need for a server-side language.
2. Ease of use: JavaScript is a relatively easy language to learn and use, especially for beginners. It has a simple syntax and provides instant feedback on code execution.
3. Cross-platform compatibility: JavaScript is a cross-platform language, which means that it can run on different operating systems and platforms. This makes it ideal for web development, as it can run on various browsers and devices.
4. Large community and resources: JavaScript has a large and active community of developers who create and share libraries, frameworks, and tools. This provides a vast pool of resources for learning and development.
5. High performance: With the advancements in JavaScript engines, the performance of JavaScript has significantly improved over the years, making it suitable for complex and resource-intensive applications.
6. Flexibility and versatility: JavaScript can be used for a wide range of applications, from simple web pages to complex web applications, mobile apps, and even server-side applications.

Overall, JavaScript’s ease of use, cross-platform compatibility, versatility, and performance make it a popular choice for web development, and its large community and resources ensure that it remains a vital and evolving language.

Q22. What are object prototypes?

In JavaScript, every object has a property called a prototype. The prototype is an object that acts as a template for the object and provides a set of default properties and methods that the object can use.

When you create an object in JavaScript, it automatically gets a prototype object assigned to it. The prototype object contains methods and properties that the object can access and use. If the object tries to access a method or property that is not defined on it, JavaScript looks for it on its prototype object and uses that instead.

Prototypes are important in JavaScript because they enable object inheritance, which is a way to create new objects based on existing ones. By setting an object’s prototype to another object, you can create a new object that inherits all the properties and methods of the parent object.

The use of prototypes makes JavaScript a powerful and flexible language that allows you to create complex and dynamic applications.

Q23. What are callbacks?

In JavaScript, a callback is a function that is passed as an argument to another function and is executed when the function is called. The purpose of using callbacks is to allow a function to call another function and continue execution without waiting for the second function to finish.

Callbacks are commonly used in asynchronous programming, where a function takes some time to complete and the program needs to continue running while waiting for the function to finish. By using a callback function, the program can continue executing while the asynchronous function runs in the background.

Callbacks can also be used for event handling, where a function is called when a particular event occurs, such as a button click or a page load.

JavaScript provides a powerful mechanism for working with callbacks, including higher-order functions such as map(), filter(), and reduce(), which take a function as an argument and apply it to each element in an array.

Callbacks are an essential concept in JavaScript and are widely used in modern web development to create interactive and dynamic applications.

Q24. What are the types of errors in javascript?

In JavaScript, there are three main types of errors: syntax errors, runtime errors, and logical errors.

1. Syntax errors: These are errors that occur when the syntax of a program is incorrect. For example, missing semicolons, brackets, or parentheses, misspelled keywords or variable names, and invalid operators can all cause syntax errors.
2. Runtime errors: These are errors that occur while a program is running. They are caused by unexpected situations, such as trying to access a variable that doesn’t exist, calling a function with the wrong number of arguments, or dividing by zero.
3. Logical errors: These are errors that occur when a program runs without any syntax or runtime errors but produces unexpected or incorrect results. These errors are caused by errors in the program’s logic or algorithms, such as using the wrong formula for a calculation or using the wrong condition in an if statement.

It’s important to understand the different types of errors in JavaScript, as each requires a different approach to debugging and fixing the issue. Syntax errors are usually easy to spot and fix, while runtime and logical errors can be more difficult to track down and resolve. Using tools such as the browser’s console and debugging tools can help you identify and fix errors in your JavaScript code.

Q25. What is memoization?

Memoization is a technique used in computer science to speed up the execution of functions by caching their results. The idea behind memoization is to avoid repeating expensive computations by storing the results of function calls and returning the cached result when the same inputs are provided again.

In JavaScript, memoization can be implemented by creating a cache object that stores the results of function calls. When a function is called, it first checks whether the result for the given inputs is already in the cache. If it is, the cached result is returned, and the function doesn’t need to be executed. If the result is not in the cache, the function is executed as usual, and the result is stored in the cache for future use.

Memoization is especially useful for functions that are computationally expensive and are called frequently with the same inputs. By caching the results of these function calls, you can improve the performance of your application and reduce the amount of time spent computing the same result multiple times.

It’s important to note that memoization should only be used for pure functions, which always return the same output for the same input and have no side effects. Memoizing impure functions can lead to unexpected behavior and should be avoided.

Q26. What is recursion in a Javascript language?

Recursion is a programming technique where a function calls itself repeatedly until it reaches a base case or termination condition. In JavaScript, recursion can be used to solve problems that can be broken down into smaller, simpler subproblems.

The basic idea behind recursion is to break down a problem into smaller subproblems, solve each subproblem recursively, and then combine the results to obtain the final solution. The recursion process continues until the base case is reached, at which point the function stops calling itself and returns a value to the calling function.

For example, the factorial of a number can be calculated using recursion:

function factorial(n) {
  if (n === 0) {
    return 1; // base case
  } else {
    return n * factorial(n - 1); // recursive call
  }
}

In this example, the factorial function calls itself with a smaller argument until it reaches the base case where n equals 0. At this point, the function stops calling itself and returns 1, which is multiplied by the previous recursive call to calculate the final result.

Recursion can be a powerful technique for solving certain types of problems, but it can also be computationally expensive and can lead to stack overflow errors if not used carefully. It’s important to ensure that the recursion process terminates at some point and to use tail recursion optimization where possible to reduce the risk of stack overflow errors.

Q27. What is the use of a constructor function in javascript?

In JavaScript, a constructor function is used to create new objects with a specific set of properties and methods. Constructor functions are defined using the function keyword and are typically named with an uppercase first letter to distinguish them from regular functions.

To create a new object using a constructor function, you use the new keyword followed by the constructor function name and any arguments that the constructor function requires. For example:

function Person(name, age) {
  this.name = name;
  this.age = age;
  this.sayHello = function() {
    console.log(`Hello, my name is ${this.name} and I'm ${this.age} years old.`);
  }
}

const person1 = new Person("John", 30);
const person2 = new Person("Jane", 25);

person1.sayHello(); // Hello, my name is John and I'm 30 years old.
person2.sayHello(); // Hello, my name is Jane and I'm 25 years old.

In this example, the Person function is used as a constructor function to create two new objects with name and age properties and a sayHello method. The new keyword is used to create new instances of the Person object, and the this keyword is used to set the name and age properties of the new object.

Constructor functions are useful because they allow you to create new objects with the same set of properties and methods without having to repeat the same code multiple times. They also provide a way to encapsulate object creation logic and make it reusable in other parts of your code.

Q28. What is DOM?

The Document Object Model (DOM) is a programming interface for web documents. It represents the page so that programs can change the document structure, style, and content. Essentially, the DOM is a hierarchical tree-like structure that represents the HTML elements and other objects on a web page.

When a web page is loaded in a browser, the browser creates a DOM for that page. Each HTML element becomes a node in the DOM tree, and JavaScript can interact with the DOM to access and manipulate these nodes.

The DOM provides a way for developers to access and modify the content and structure of a web page dynamically using JavaScript. For example, you can use the DOM to:

• Add or remove HTML elements from a page
• Change the content of HTML elements
• Change the styles of HTML elements
• React to user interactions (e.g., clicking a button)

The DOM is a crucial part of web development, as it enables dynamic web applications and rich user interfaces.

Q29. Which method is used to retrieve a character from a certain index?

In JavaScript, you can retrieve a character from a certain index of a string using the charAt() method. This method takes an index as its argument and returns the character at that index.

Here’s an example:

let str = 'Hello, World!';
let char = str.charAt(7); // returns 'W'

In this example, we create a string str and then use the charAt() method to retrieve the character at index 7, which is ‘W’. Note that string indexes start at 0, so the first character in the string has an index of 0, the second character has an index of 1, and so on.

Q30. What do you mean by BOM?

BOM stands for Browser Object Model, which is a collection of objects and APIs that provide access to the browser’s window and its components. The BOM is specific to web browsers and is not a part of the core JavaScript language.

The BOM includes objects like:

• window: The global object that represents the browser window.
• navigator: An object that provides information about the user’s browser.
• location: An object that provides information about the current URL and can be used to navigate to other URLs.
• history: An object that provides access to the user’s browsing history.
• screen: An object that provides information about the user’s screen resolution and color depth.

The BOM also includes APIs for working with browser windows and frames, cookies, and client-side storage.

It’s important to note that the BOM is not standardized and may vary across different browsers. This means that code that relies on specific BOM features may not work consistently across different browsers.

Q31. What is the distinction between client-side and server-side JavaScript?

Client-side JavaScript runs on the user’s web browser, while server-side JavaScript runs on a server.

Client-side JavaScript is used to add interactivity and dynamic behavior to web pages. It can be used to manipulate the Document Object Model (DOM) of a web page, handle user events, perform form validation, make asynchronous HTTP requests, and more. Client-side JavaScript code is downloaded from the web server along with the HTML, CSS, and other resources required to render a web page. Once downloaded, the code runs in the user’s browser, allowing for a rich and interactive user experience.

Server-side JavaScript is used to build dynamic web applications, where the server generates dynamic HTML or responds to API requests from client-side JavaScript code. Server-side JavaScript is often used in combination with Node.js, a JavaScript runtime built on the V8 engine, which allows developers to write server-side JavaScript code using the same language and tools as client-side JavaScript.

In summary, client-side JavaScript runs in the user’s web browser and is used to add interactivity and dynamic behavior to web pages, while server-side JavaScript runs on a server and is used to build dynamic web applications.

Q32. What are arrow functions?

Arrow functions are a shorthand way of writing function expressions in JavaScript. They were introduced in ECMAScript 6 and provide a more concise syntax for defining functions.

Here is an example of a traditional function expression:

let sum = function(a, b) {
  return a + b;
}

And here is the equivalent arrow function expression:

let sum = (a, b) => a + b;

Arrow functions have several features that make them useful:

1. Concise syntax: Arrow functions have a shorter syntax than traditional function expressions, which can make code easier to read and write.
2. Implicit return: If an arrow function has a single expression, the result of that expression is automatically returned. This can make code more concise and easier to read.
3. Lexical this: Arrow functions do not have their own this value, but instead inherit the this value from the surrounding code. This can help avoid some of the confusion that can arise with traditional functions when dealing with the this keyword.

Overall, arrow functions are a useful tool for writing more concise and readable JavaScript code.

Q33. What do mean by prototype design pattern?

In JavaScript, the Prototype Design Pattern is a way of creating objects that can serve as blueprints for other objects. The basic idea is that you create a prototype object that contains all of the properties and methods that other objects will need, and then you create new objects that inherit those properties and methods from the prototype.

Here’s an example:

// Create a prototype object
var animal = {
  name: "",
  sound: "",
  speak: function() {
    console.log(this.name + " says " + this.sound);
  }
};

// Create a new object that inherits from the prototype
var cat = Object.create(animal);
cat.name = "Fluffy";
cat.sound = "Meow";

// Use the inherited method to make the cat speak
cat.speak(); // Output: "Fluffy says Meow"

In this example, we create a prototype object called animal that has a name property, a sound property, and a speak method. Then we create a new object called cat that inherits from the animal prototype. We set the name and sound properties of the cat object, and then we call the speak method, which is inherited from the prototype.

The Prototype Design Pattern can be very useful in JavaScript because it allows you to create objects that share common properties and methods without having to duplicate code. It also makes it easy to create new objects that have slightly different behavior from their prototypes, by simply adding or overriding properties and methods.

Q34. Differences between declaring variables using var, let and const.

In JavaScript, there are three ways to declare variables: var, let, and const. While all three are used to declare variables, they differ in terms of their scope, hoisting, and mutability.

1. var: Variables declared with var have function scope or global scope, depending on where they are declared. They are also hoisted to the top of their scope, which means that they can be used before they are declared. var variables can be reassigned and updated.

Example:

function example() {
  var x = 1;
  if (true) {
    var x = 2; // same variable as above
    console.log(x); // Output: 2
  }
  console.log(x); // Output: 2
}

3. const: Variables declared with const have block scope and cannot be reassigned. They must be initialized when they are declared, and cannot be updated later.

Example:

function example() {
  const x = 1;
  if (true) {
    const x = 2; // different variable than above
    console.log(x); // Output: 2
  }
  console.log(x); // Output: 1
}

In summary, var is used for variables that need to be accessible across the entire function or global scope, let is used for variables that need to be limited to a particular block scope, and const is used for variables that should not be reassigned.

Q35. What is the rest parameter and spread operator?

In JavaScript, the rest parameter and spread operator are two features that were introduced in ES6 (ECMAScript 2015) to make working with arrays and functions easier.

The rest parameter is used to represent an indefinite number of arguments as an array. It allows you to pass any number of arguments to a function and store them in an array.

Here is an example:

function myFunction(...args) {
  console.log(args);
}

myFunction(1, 2, 3); // output: [1, 2, 3]

The spread operator, on the other hand, is used to spread the elements of an array or an object into individual elements. It allows you to convert an array into a list of arguments or add the elements of one array into another.

Here is an example:

const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];

const arr3 = [...arr1, ...arr2];

console.log(arr3); // output: [1, 2, 3, 4, 5, 6]

In the above example, we use the spread operator to concatenate the elements of arr1 and arr2 into a new array arr3.

The rest parameter and spread operator are powerful features that can simplify your code and make it more readable.

Q36. In JavaScript, how many different methods can you make an object?

In JavaScript, there are several ways to create objects:

1. Object literals: Objects can be created using object literals which is the most common way. Object literals are created using curly braces {} and are used to create and initialize an object at the same time.

Example:

let person = {
  name: 'John',
  age: 30,
  city: 'New York'
};

2. Constructor functions: You can create objects using constructor functions, which are functions that are used to create new instances of objects. The new keyword is used to create a new instance of an object.

Example:

function Person(name, age, city) {
  this.name = name;
  this.age = age;
  this.city = city;
}

let person1 = new Person('John', 30, 'New York');

3. Object.create method: This method is used to create a new object and assign an existing object as the new object’s prototype.

Example:

let person = {
  name: 'John',
  age: 30,
  city: 'New York'
};

let person1 = Object.create(person);

4. ES6 Classes: In ES6, you can create objects using classes, which are a syntactical sugar over constructor functions.

Example:

class Person {
  constructor(name, age, city) {
    this.name = name;
    this.age = age;
    this.city = city;
  }
}

let person1 = new Person('John', 30, 'New York');

These are the most common ways to create objects in JavaScript.

Q37. What is the use of promises in javascript?

Promises are used in JavaScript for asynchronous programming, where a code block is executed without blocking the main thread, and a result is returned once it’s ready. Promises provide a way to handle asynchronous operations, such as API requests, database queries, or file system operations, in a more readable and organized way.

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It has three states:

1. Pending: The initial state; neither fulfilled nor rejected.
2. Fulfilled: Meaning that the operation completed successfully.
3. Rejected: Meaning that the operation failed.

Promises have two main methods:

1. then(): This method is called when a Promise is resolved, and it takes two parameters: a success callback and an optional error callback. The success callback is executed when the Promise is fulfilled, while the error callback is executed when the Promise is rejected.
2. catch(): This method is called when a Promise is rejected, and it takes an error callback as its parameter. The error callback is executed when the Promise is rejected.

Promises can also be chained using then(), which allows for more complex asynchronous operations to be performed in a readable and organized manner.

Q38. What are classes in javascript?

Classes in JavaScript are a syntactical feature that allows creating objects based on a template, similar to class-based object-oriented programming languages like Java or C++. Classes in JavaScript are based on the prototype-based object-oriented programming model that JavaScript supports.

Classes are declared using the class keyword and can have constructors, methods, and properties defined within them. Class-based objects can be created using the new keyword, which creates an instance of the class.

Here is an example of a simple class in JavaScript:

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  sayHello() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

let person = new Person("John", 25);
person.sayHello(); // Output: "Hello, my name is John and I am 25 years old."

In this example, the Person class has a constructor that sets the name and age properties of the object. It also has a sayHello method that logs a message to the console. An instance of the Person class is created using the new keyword, and the sayHello method is called on the instance.

Q39. What are generator functions?

Generator functions are a type of function in JavaScript that can be paused and resumed during their execution, allowing for the generation of a series of values over time. When a generator function is called, it does not immediately execute the entire function body, but instead returns a special object called a generator object. This object has a next() method that can be called to execute the next step of the generator function.

The key feature of generator functions is that they can use the yield keyword to pause their execution and return a value to the caller. When the next() method is called on the generator object, execution of the generator function resumes from the point where it was paused by the previous yield keyword. This allows the generator function to generate a series of values on demand, rather than having to generate all of the values up front.

Generator functions are particularly useful for generating large datasets that would be too memory-intensive to generate all at once, or for implementing asynchronous operations that need to be performed in a specific order.

Q40. Explain WeakSet in javascript.

n JavaScript, a WeakSet is a built-in object that allows you to store weakly held object references in a collection. It is similar to a Set, but with a few key differences.

Here are some important characteristics of a WeakSet:

• A WeakSet can only contain object references (not primitive values).
• Objects inside a WeakSet can be garbage collected even if the WeakSet still contains a reference to them.
• Unlike a Set, a WeakSet does not have a size property and cannot be iterated over.

Here’s an example of using a WeakSet:

let mySet = new WeakSet();

let obj1 = { name: 'John' };
let obj2 = { name: 'Mary' };
let obj3 = { name: 'Bob' };

mySet.add(obj1);
mySet.add(obj2);
mySet.add(obj3);

console.log(mySet.has(obj1)); // true

// garbage collect obj2
obj2 = null;

console.log(mySet.has(obj2)); // false

In this example, we create a WeakSet and add three objects to it. We then check if the set contains obj1 (which it does), and then set obj2 to null to simulate garbage collection. After obj2 is garbage collected, we check if the set contains obj2 (which it does not).

Q41. Why do we use callbacks?

In JavaScript, callbacks are used to handle asynchronous operations such as API calls, event handling, and timeouts. When an asynchronous operation is initiated, it is executed outside the main program flow. Once the operation is completed, the callback function is executed to handle the results or errors. Callbacks allow us to ensure that the main program flow does not wait for an asynchronous operation to complete, preventing the application from freezing or becoming unresponsive.

Callbacks are also useful in situations where we want to execute a function after another function has finished executing. For example, if we want to log a message after a specific function is called, we can pass a callback function to that function and execute it once the function has finished executing.

Q42. Explain WeakMap in javascript.

In JavaScript, a WeakMap is a type of map that allows objects to be used as keys. Unlike a regular Map, the keys in a WeakMap are weakly referenced, which means that they do not prevent the garbage collector from cleaning up the object if there are no other references to it. In other words, if an object used as a key in a WeakMap is no longer being used by the rest of the program, it will automatically be removed from the WeakMap.

Here’s an example of how to use a WeakMap:

// Create a WeakMap object
const weakMap = new WeakMap();

// Create an object to use as a key
const key = {};

// Add a key-value pair to the WeakMap
weakMap.set(key, "value");

// Get the value associated with the key
console.log(weakMap.get(key)); // Output: "value"

// Remove the key from the WeakMap
weakMap.delete(key);

In the example above, we first create a WeakMap object using the new keyword. Then, we create an object called key to use as a key in the WeakMap. We add a key-value pair to the WeakMap using the set() method, and then we retrieve the value associated with the key using the get() method. Finally, we remove the key from the WeakMap using the delete() method.

One thing to note is that the WeakMap only accepts objects as keys, whereas a regular Map can accept any type of value as a key. Additionally, since the keys in a WeakMap are weakly referenced, they cannot be iterated over or counted like a regular Map.

WeakMap can be useful in cases where you want to associate some data with an object, but you don’t want to keep the object in memory if it’s no longer being used. For example, you might use a WeakMap to store some metadata about an object in a way that doesn’t interfere with garbage collection.

Q43. What is Object Destructuring?

Object destructuring is a feature in JavaScript that allows you to extract properties from an object and assign them to variables in a concise and easy-to-read way. It provides a convenient syntax for accessing and assigning values within objects.

Here’s an example of how to use object destructuring:

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 30,
  address: {
    street: "123 Main St",
    city: "Anytown",
    state: "CA",
    zipCode: "12345"
  }
};

// Destructure the person object
const { firstName, lastName, age, address: { city } } = person;

// Output the destructured variables
console.log(firstName); // Output: "John"
console.log(lastName); // Output: "Doe"
console.log(age); // Output: 30
console.log(city); // Output: "Anytown"

In the example above, we have an object called person that contains several properties, including a nested address object. We can use object destructuring to extract the values of the firstName, lastName, age, and city properties and assign them to variables with the same name. We also use object destructuring to extract the city property from the nested address object and assign it to a variable with the name city.

Object destructuring can also be used with function parameters, like this:

// Define a function that takes an object as a parameter
function printPersonInfo({ firstName, lastName, age }) {
  console.log(`Name: ${firstName} ${lastName}, Age: ${age}`);
}

// Call the function and pass in an object
printPersonInfo({ firstName: "John", lastName: "Doe", age: 30 });
// Output: "Name: John Doe, Age: 30"

In the example above, we define a function called printPersonInfo that takes an object as a parameter. We use object destructuring to extract the firstName, lastName, and age properties from the object and use them to output a string. When we call the function and pass in an object, the destructured variables are automatically assigned values based on the object’s properties.

Q44. Difference between prototypal and classical inheritance

JavaScript supports both prototypal and classical inheritance.

Classical inheritance is based on the concept of classes, which are blueprints for creating objects. In classical inheritance, you define a class with properties and methods, and then create instances of that class. The instances inherit the properties and methods of the class. In JavaScript, classical inheritance is implemented using constructor functions and the new keyword.

Prototypal inheritance, on the other hand, is based on the concept of prototypes. In prototypal inheritance, you create an object that serves as a prototype, and then create new objects that inherit from that prototype. The new objects can have their own properties and methods, but they also inherit properties and methods from the prototype.

Here are some key differences between prototypal and classical inheritance in JavaScript:

1. Syntax: In classical inheritance, you define a class using a constructor function, and then create instances of that class using the new keyword. In prototypal inheritance, you create an object that serves as a prototype, and then create new objects that inherit from that prototype using the Object.create() method.
2. Inheritance: In classical inheritance, a child class inherits properties and methods from its parent class. In prototypal inheritance, a child object inherits properties and methods from its prototype.
3. Multiple Inheritance: Classical inheritance allows for multiple inheritance, where a class can inherit from multiple parent classes. Prototypal inheritance does not support multiple inheritance directly, but it can be simulated by chaining prototypes.
4. Dynamic Typing: JavaScript is a dynamically typed language, which means that the type of a variable can change at runtime. In classical inheritance, the type of an object is determined by its class. In prototypal inheritance, the type of an object is determined by its prototype.

In summary, classical inheritance is based on the concept of classes and uses constructor functions and the new keyword to create instances of classes. Prototypal inheritance is based on the concept of prototypes and uses the Object.create() method to create objects that inherit from a prototype. Prototypal inheritance is simpler and more flexible than classical inheritance, and it is the preferred method of inheritance in JavaScript.

Q45. What is a Temporal Dead Zone?

In JavaScript, the Temporal Dead Zone (TDZ) is a behavior that occurs when trying to access a variable before it has been initialized. The TDZ is a time period between the creation of a variable and the point at which it is initialized. During this time period, any attempt to access the variable will result in a ReferenceError.

The TDZ occurs because of the way that the let and const declarations work. When a variable is declared with let or const, the variable is created in memory but is not initialized until the declaration is reached in the code. However, the variable is still in scope and can be accessed, but any attempt to access it before it is initialized will result in a ReferenceError.

Here’s an example of the TDZ in action:

console.log(x); // Output: ReferenceError: Cannot access 'x' before initialization
let x = 10;

In this example, we try to access the x variable before it has been initialized. Since the variable is declared with let, it is created in memory but is not initialized until the let declaration is reached in the code. The attempt to access the variable before it has been initialized results in a ReferenceError.

The TDZ is important to understand because it can help you write better code. It’s a good practice to declare your variables at the beginning of a block or function to avoid the TDZ. Additionally, understanding the TDZ can help you identify and fix bugs in your code where you may be trying to access a variable before it has been initialized.

Q46. What do you mean by JavaScript Design Patterns?

JavaScript Design Patterns are reusable solutions to common problems that developers face while designing and developing applications in JavaScript. Design patterns provide a standardized approach to solve problems in a specific context. They are proven solutions to recurring problems that developers encounter while developing software applications.

JavaScript design patterns are based on the principles of object-oriented programming (OOP) and can be used to create robust, scalable, and maintainable applications. Some of the benefits of using design patterns in JavaScript include:

1. Encapsulation: Design patterns promote encapsulation, which means that the implementation details of a module or object are hidden from the outside world. This helps to keep the codebase organized and makes it easier to maintain and modify.
2. Reusability: Design patterns provide reusable solutions to common problems, which can save time and effort in developing new applications.
3. Scalability: Design patterns are scalable and can be used to develop large, complex applications.
4. Maintainability: Design patterns promote modular and organized code, which makes it easier to maintain and modify the codebase.

Some common JavaScript design patterns include:

1. Module Pattern
2. Revealing Module Pattern
3. Singleton Pattern
4. Factory Pattern
5. Observer Pattern
6. Mediator Pattern
7. Command Pattern
8. Prototype Pattern
9. Decorator Pattern

Understanding and implementing design patterns can make your JavaScript code more maintainable, modular, and scalable. It can also help you to write better code and avoid common pitfalls in software development.

Q47. Is JavaScript a pass-by-reference or pass-by-value language?

JavaScript is a pass-by-value language. However, it is important to understand that the value being passed may be a reference to an object.

When primitive data types (such as numbers, booleans, and strings) are passed as arguments to a function, the value is copied to a new memory location and passed to the function. This means that changes made to the argument within the function do not affect the original value.

For example:

function myFunction(x) {
  x = 10;
  console.log(x);
}

let y = 5;
myFunction(y); // Output: 10
console.log(y); // Output: 5

In this example, the value of y is passed to myFunction. When x is set to 10 within the function, it does not affect the original value of y outside of the function.

When objects are passed as arguments to a function, the value being passed is a reference to the object, rather than a copy of the object itself. This means that changes made to the object within the function will affect the original object.

For example:

function myFunction(obj) {
  obj.x = 10;
  console.log(obj);
}

let myObj = {x: 5};
myFunction(myObj); // Output: {x: 10}
console.log(myObj); // Output: {x: 10}

In this example, the myObj object is passed to myFunction. When obj.x is set to 10 within the function, it affects the original myObj object outside of the function.

So, while JavaScript is a pass-by-value language, objects can be thought of as being passed by reference due to the reference to the object being passed.

Q48. Difference between Async/Await and Generators usage to achieve the same functionality.

Both Async/Await and Generators can be used to achieve similar functionality, which is to manage asynchronous operations in JavaScript. However, they have different syntax and ways of handling asynchronous operations.

Async/Await is a language feature introduced in ES2017 that provides a way to write asynchronous code that looks and behaves like synchronous code. It uses the async keyword to mark a function as asynchronous and the await keyword to wait for the completion of an asynchronous operation. When an await keyword is encountered in an async function, the function is paused until the asynchronous operation is complete.

For example:

async function myFunction() {
  const result1 = await fetch('https://example.com/data1');
  const result2 = await fetch('https://example.com/data2');
  return [result1, result2];
}

In this example, the myFunction function is marked as asynchronous with the async keyword. Two asynchronous operations are performed using the fetch function, and the results are obtained using the await keyword. The function returns an array of the two results.

Generators, on the other hand, are functions that can be paused and resumed using the yield keyword. They provide a way to write code that can be paused and resumed at specific points, which can be used to manage asynchronous operations.

For example:

function* myGenerator() {
  const result1 = yield fetch('https://example.com/data1');
  const result2 = yield fetch('https://example.com/data2');
  return [result1, result2];
}

In this example, the myGenerator function is a generator function that yields the results of two asynchronous operations. The function can be paused and resumed using the yield keyword.

While both Async/Await and Generators can be used to manage asynchronous operations, Async/Await is generally considered to be easier to read and write because it uses a more straightforward syntax. However, Generators provide more fine-grained control over the asynchronous operations and can be used in more complex scenarios.

Q49. What are the primitive data types in JavaScript?

In JavaScript, there are six primitive data types:

1. Boolean: The Boolean data type represents a value that can be either true or false. For example, true and false are Boolean values.
2. Number: The Number data type represents a numeric value, including integers and floating-point numbers. For example, 42 and 3.14 are numeric values.
3. String: The String data type represents a sequence of characters. Strings are enclosed in quotes, either single or double. For example, "Hello" and 'World' are strings.
4. Null: The Null data type represents a deliberate non-value. It is typically used to indicate the absence of an object or value. For example, null can be used to represent a missing value.
5. Undefined: The Undefined data type represents an uninitialized or non-existent value. If a variable has not been assigned a value, it is considered to be undefined.
6. Symbol: The Symbol data type represents a unique identifier. Symbols are often used to create object properties that are guaranteed to be unique. Symbols are created using the Symbol() function.

These primitive data types are immutable, which means that their values cannot be changed once they are created. Any operation that appears to modify a primitive value actually creates a new value rather than modifying the original value.

Q50. What is the role of deferred scripts in JavaScript?

Deferred scripts are a way to improve the performance of a web page by loading scripts asynchronously while ensuring that they execute in the correct order.

When a script is marked as “defer”, it tells the browser to download the script in the background, without blocking the rendering of the page. The deferred script will be executed after the HTML document has been parsed, but before the DOMContentLoaded event is fired. This means that the script will be executed after the page has been fully loaded and rendered, but before the user can interact with the page.

The main benefit of using deferred scripts is that they do not block the rendering of the page, which can improve the perceived speed of the page. This is especially important for scripts that are not critical to the initial rendering of the page, such as analytics or advertising scripts.

It is important to note that deferred scripts are still executed in the order in which they appear in the HTML document. This means that if there are dependencies between scripts, they should be included in the correct order to ensure that they are executed in the correct order.

To mark a script as “deferred”, the defer attribute can be added to the <script> tag, like this:

<script src="my-script.js" defer></script>

It is also possible to combine the defer attribute with the async attribute, which tells the browser to load the script asynchronously if possible, but to defer execution until after the HTML document has been parsed.

<script src="my-script.js" async defer></script>

Q51. What has to be done in order to put Lexical Scoping into practice?

To put Lexical Scoping into practice in JavaScript, the following steps need to be taken:

1. Define a function: Lexical scoping is based on the concept of nested functions. To create a lexical scope, we first need to define a function.
2. Create a nested function: Within the outer function, we can create one or more nested functions. These nested functions will have access to the variables in the outer function’s scope.
3. Use the nested function: Once we have defined the nested function, we can use it within the outer function. When the nested function is called, it will have access to the variables in the outer function’s scope, as well as its own local variables.

Here is an example of using lexical scoping in JavaScript:

function outer() {
  let message = 'Hello, ';

  function inner(name) {
    console.log(message + name);
  }

  return inner;
}

let sayHello = outer();
sayHello('John'); // Output: Hello, John

In this example, the outer function defines a nested function called inner. The inner function has access to the message variable in the outer function’s scope, as well as its own name parameter. The outer function returns the inner function, which can then be called and passed a name argument. When the inner function is called with a name argument, it logs a message to the console that includes the message variable and the name argument.

Note that lexical scoping allows the inner function to access the message variable even after the outer function has returned. This is because the inner function “closes over” the message variable, creating a closure that persists even after the outer function has finished executing.

Q52. What is the purpose of the following JavaScript code?

var scope = "global scope";
function check() 
{
    var scope = "local scope"; 
    function f() 
    { 
         return scope; 
    }
    return f;
}

The purpose of the given JavaScript code is to demonstrate the concept of lexical scoping and closures in JavaScript.

The code defines a variable scope in the global scope and initializes it with the string “global scope”. It then defines a function check which, in turn, defines a variable scope in its local scope and initializes it with the string “local scope”. The function check also defines a nested function f that returns the value of the scope variable. Finally, the check function returns the f function.

When the check function is called, it creates a closure that captures the local variable scope and the nested function f. The returned function f is itself a closure that has access to the captured scope variable, even after the check function has completed execution.

We can use the returned function f to access the value of the scope variable that was captured at the time of the check function’s execution:

var scope = "global scope";
var f = check(); // f is now a closure that captures the "local scope" variable
console.log(f()); // Output: "local scope"

When we call the f function, it returns the value of the scope variable that was captured at the time of the check function’s execution, which is “local scope”. This demonstrates how closures and lexical scoping work together in JavaScript.

// Code 1:

function func1(){
  setTimeout(()=>{
    console.log(x);
    console.log(y);
  },3000);

  var x = 2;
  let y = 12;
}
func1();

// Code 2:

function func2(){
  for(var i = 0; i < 3; i++){
    setTimeout(()=> console.log(i),2000);
}
}
func2();

// Code 3:

(function(){
  setTimeout(()=> console.log(1),2000);
  console.log(2);
  setTimeout(()=> console.log(3),0);
  console.log(4);
})();

Code 1:

function func1(){
  setTimeout(()=>{
    console.log(x);
    console.log(y);
  },3000);

  var x = 2;
  let y = 12;
}
func1();

undefined
12

function func2(){
  for(var i = 0; i < 3; i++){
    setTimeout(()=> console.log(i),2000);
  }
}
func2();

3
3
3

(function(){
  setTimeout(()=> console.log(1),2000);
  console.log(2);
  setTimeout(()=> console.log(3),0);
  console.log(4);
})();

2
4
3
1

// Code 1:

let x= {}, y = {name:"Ronny"},z = {name:"John"};
x[y] = {name:"Vivek"};
x[z] = {name:"Akki"};
console.log(x[y]);

// Code 2:

function runFunc(){
  console.log("1" + 1);
  console.log("A" - 1);
  console.log(2 + "-2" + "2");
  console.log("Hello" - "World" + 78);
  console.log("Hello"+ "78");
}
runFunc();

// Code 3:

let a = 0;
let b = false;
console.log((a == b));
console.log((a === b));

var x = 23;

(function(){
  var x = 43;
  (function random(){
    x++;
    console.log(x);
    var x = 21;
  })();
})();

NaN