Top 10 JavaScript Interview Questions

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What is the purpose of the “use strict” directive in JavaScript?
What is event delegation in JavaScript?
What is the difference between the “==” and “===” operators in JavaScript?
What is closure in JavaScript?
What is the difference between a for loop and a for…in loop in JavaScript?
What is the difference between let and var in JavaScript?
What is hoisting in JavaScript?
What is closure in JavaScript?
What is the difference between synchronous and asynchronous JavaScript code?
What is the difference between null and undefined in JavaScript?

What is the purpose of the “use strict” directive in JavaScript?

Response: The “use strict” directive is used to enable strict mode in JavaScript, which enforces stricter parsing and error handling rules.

Explanation: This question tests the candidate’s understanding of strict mode in JavaScript. A good response should explain what strict mode is and provide examples of how it affects JavaScript programming.

What is event delegation in JavaScript?

Response: Event delegation is a technique in JavaScript where an event listener is added to a parent element to listen for events that occur on its child elements.

Explanation: This question tests the candidate’s understanding of event handling in JavaScript. A good response should explain what event delegation is and provide examples of how it is used in JavaScript programming.

What is the difference between the “==” and “===” operators in JavaScript?

Response: The “==” operator performs type coercion and compares two values for equality, while the “===” operator compares two values for equality without type coercion.

Explanation: This question tests the candidate’s understanding of comparison operators in JavaScript. A good response should explain the differences between “==” and “===” and provide examples of when each operator should be used.

What is closure in JavaScript?

Response: Closure is a function that has access to its outer function’s variables, even after the outer function has returned.

Explanation: This question tests the candidate’s understanding of closures in JavaScript. A good response should explain how closures work and provide examples of when they are used in JavaScript programming.

What is the difference between a for loop and a for…in loop in JavaScript?

Response: A for loop is used to iterate over an array or a specified number of times, while a for…in loop is used to iterate over the properties of an object.

Explanation: This question tests the candidate’s understanding of loop structures in JavaScript. A good response should explain the differences between for and for…in loops and provide examples of when each loop should be used.

What is the difference between let and var in JavaScript?

Response: The let keyword was introduced in ES6 and is used to declare block-scoped variables, while the var keyword declares variables that are function-scoped.

Explanation: This question tests the candidate’s understanding of variable declaration in JavaScript. A good response should explain the differences between let and var and provide examples of when to use each keyword.

What is hoisting in JavaScript?

Response: Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their respective scopes during compilation, allowing them to be accessed before they are declared.

Explanation: This question tests the candidate’s understanding of variable and function declaration in JavaScript. A good response should explain what hoisting is and provide examples of how it can affect JavaScript programming.

What is closure in JavaScript?

Response: A closure is a function that has access to variables in its outer (enclosing) function scope, even after the outer function has returned.

Explanation: This question tests the candidate’s understanding of closures in JavaScript. A good response should explain what a closure is and provide examples of how it is used in JavaScript programming.

What is the difference between synchronous and asynchronous JavaScript code?

Response: Synchronous JavaScript code executes one line of code at a time, blocking other code from running until it has completed, while asynchronous JavaScript code can run multiple lines of code at the same time, allowing other code to run in the meantime.

Explanation: This question tests the candidate’s understanding of synchronous and asynchronous programming in JavaScript. A good response should explain the differences between synchronous and asynchronous code and provide examples of when each programming model should be used.

What is the difference between null and undefined in JavaScript?

Response: Undefined is a value that is assigned to a variable that has not been initialized, while null is an explicitly assigned value that represents the absence of any object value.

Explanation: This question tests the candidate’s understanding of null and undefined values in JavaScript. A good response should explain the differences between null and undefined and provide examples of when to use each value.