Essential JavaScript Interview Questions and Answers

JavaScript Interview Questions and Answers

Here we have some JavaScript Interview Questions and Answers which can be asked. These JavaScript Questions are asked to beginners, who are starting their careers in JavaScript Programming.

Question 1: What is this keyword?

The this keyword in JavaScript refers to the object of which the function is a property. It represents the context in which the current code is being executed. In the global context, this refers to the global object (e.g., window in a web browser or the global object in Node.js).

Question 2: What is a global variable?

A global variable is a variable that is declared outside any function or declared with the window object. It can be accessed from any function throughout the program.

Question 3: What is a window object?

The window object is a global object supported by all browsers. It represents the browser's window and serves as the global context for JavaScript in a web browser. All global JavaScript objects, functions, and variables automatically become members of the window object. Global variables are properties of the window object.

Question 4: What is a true or false value in Javascript?

In JavaScript, a true value is a value that translates to true when evaluated in a Boolean context. Conversely, a false value is a value that translates to false when evaluated in a Boolean context. Some examples of falsy values in JavaScript include:

• 0

• false

• an empty string ('')

• undefined

• null

• NaN

All other values in JavaScript are considered true unless they are explicitly defined as false.

Question 5: What is the difference between null and undefined in JavaScript?

In JavaScript, undefined is a variable that has been declared but has no value assigned to it. For example:

const person;
console.log(person);
// Output: undefined

Functions that do not have a return statement implicitly return undefined. For example:

const add = (a, b) => {
  const result = a + b;
  return;
};

let result = add(10, 20);
console.log(result);
// Output: undefined

On the other hand, null represents the intentional absence of any object value. It must be assigned explicitly. For example:

const person = null;
console.log(person);
// Output: null

Question 6: What is the difference between double equal (==) and triple equal (===) in JavaScript?

The double equals (==) is an abstract equality comparison operator in JavaScript. It coerces the operands to the same type before making the comparison. For example:

4 == 4 // Output: true'
4' == 4 // Output: true

On the other hand, the triple equals (===) is a strict equality comparison operator. It not only compares the values but also checks for the same type. It returns false for different types and different content. For example:

4 === 4 // Output: true
4 === '4' // Output: false

Question 7: What is the scope of JavaScript?

Scope in JavaScript determines the visibility and accessibility of a variable. There are three types of scope in JavaScript:

• Local scope: Variables declared inside a function are only accessible within that function.

• Global scope: Variables declared outside of any function are accessible globally throughout the program.

• Block scope: Introduced in ES6 with the let and const keywords, variables declared within a block (e.g., inside an if statement or a loop) are only accessible within that block.

Example:

function sum(first, second) {
  let result = first + second;
  return result;
}

const output = sum(2, 4);
console.log(output);
// Output: 6
console.log(result);
// Output: ReferenceError: result is not defined

Question 8: What is closure in JavaScript?

In JavaScript, a closure is a function that has access to variables from its outer (enclosing) scope, even after the outer function has finished executing. The closure "closes over" the variables, preserving their values within its own scope.

Example:

function makeFunc() {
  var name = 'Mozilla';
  function displayName() {
    alert(name);
  }
  return displayName;
}

var myFunc = makeFunc();
myFunc();

In the above example, the inner function displayName has access to the name variable even after the makeFunc function has finished executing.

Question 9: What is JavaScript Encapsulation?

JavaScript encapsulation is a process of binding data with the functions acting on that data. It allows us to control the data and validate it. To achieve encapsulation in JavaScript:

• Use the var keyword to make data members private.

• Use setter methods to set the data and getter methods to retrieve the data.

Question 10: What are call(), apply(,) and bind() in JavaScript?

call(): The call() method is used to call a function with a specified this value and individual arguments provided. It allows borrowing methods from other objects.

Example:

function Product(name, price) {
  this.name = name;
  this.price = price;
}

function Fruits(name, price) {
  Product.call(this, name, price);
  this.category = 'fruits';
}

console.log(new Fruits('mango', 5).name);
// Output: "mango"

apply(): The apply() method is similar to call(), but it accepts an array-like object as the second argument, where each element of the array is passed as an individual argument to the function.

Example:

const numbers = [5, 6, 2, 3, 7];
const max = Math.max.apply(null, numbers);
console.log(max);
// Output: 7
const min = Math.min.apply(null, numbers);
console.log(min);
// Output: 2

bind(): The bind() method is used to create a new function with a specified this value. It allows setting the this value and, optionally, pre-filling arguments.

Question 11: What is the difference between let, const, and var in JavaScript?

• let: Variables declared with let have block scope and can be reassigned.

• const: Variables declared with const are also block-scoped but cannot be reassigned once a value is assigned.

• var: Variables declared with var are function-scoped and can be reassigned. They are also hoisted to the top of their scope.

Question 12: What is event delegation in JavaScript?

Event delegation is a technique where you attach a single event listener to a parent element instead of attaching multiple event listeners to individual child elements. This allows you to handle events on dynamically added or removed elements efficiently.

Question 13: What is the difference between null and undefined?

• null represents the intentional absence of any object value. It must be assigned explicitly.

• undefined means a variable has been declared but has no value assigned to it. It is the default value for uninitialized variables and the return value of functions without an explicit return statement.

Question 14: Explain the concept of hoisting in JavaScript.

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase. This allows you to use variables and call functions before they are declared in the code.

Question 15: What is the event loop in JavaScript?

The event loop is a mechanism in JavaScript that handles asynchronous operations. It continuously checks the call stack and the task queue. When the call stack is empty, it picks tasks from the task queue and adds them to the call stack for execution.