Here are some common interview questions that you might be asked during a JavaScript job interview, along with suggested answers:

  1. What is the difference between let and var in JavaScript?
  • let is a block-scoped variable, which means it is only accessible within the block it is defined in. var is a function-scoped variable, which means it is accessible within the entire function in which it is defined.
  1. What is closure in JavaScript?
  • A closure is a function that has access to variables in its outer scope, even after the outer function has returned.
  1. Can you explain how event bubbling and event capturing work in JavaScript?
  • Event bubbling means that when an event is triggered on a child element, it bubbles up to its parent element and triggers the same event on that element. Event capturing is the opposite process, where the event is first captured by the outermost element and then propagates down to the innermost element.
  1. What is the difference between == and === in JavaScript?
  • == is a loose comparison, meaning it compares the values of the operands without considering their type. === is a strict comparison, meaning it compares the values and the types of the operands.
  1. Can you explain how the this keyword works in JavaScript?
  • In JavaScript, this keyword refers to the object that the function is a method of. In other words, it refers to the object on which the function is invoked. Its value is determined by the way the function is called.
  1. What is hoisting in JavaScript?
  • Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their scope. This means that variables and functions can be used before they are declared in the code.
  1. What is the difference between null and undefined in JavaScript?
  • undefined is a value that a variable can be assigned when it has been declared but has not been assigned a value. null is a value that is explicitly assigned to a variable to represent no value.
  1. Can you explain the difference between a forEach loop and a map function in JavaScript?
  • A forEach loop is used to iterate over an array and perform a specific operation on each element of the array. A map function is also used to iterate over an array, but it creates a new array as a result, with each element transformed according to the operation specified in the function.
  1. What is a promise in JavaScript?
  • A promise is a JavaScript object that represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.
  1. How would you go about debugging JavaScript code?
  • By using browser’s developer tools, setting breakpoints, stepping through the code, adding console.logs, using debugger statement, or using frameworks and libraries like React developer tools, Vue devtools, and lots more.
  1. Can you explain the difference between synchronous and asynchronous code in JavaScript?
  • Synchronous code is code that runs one statement at a time, in the order that it appears in the code. Asynchronous code is code that runs in a non-blocking manner, allowing other code to run at the same time.
  1. How do you handle errors in JavaScript?
  • JavaScript provides a try-catch statement for handling errors. The try block contains the code that may throw an error, and the catch block contains the code that will handle the error.
  1. How do you create a class and inherit properties in JavaScript?
  • JavaScript uses the class keyword to define a class and the extends keyword to inherit properties from a parent class.
  1. How do you implement a module system in JavaScript?
  • JavaScript uses the export and import keywords to create and use modules.
  1. How would you implement a debounce function in JavaScript?
  • A debounce function is used to limit the rate at which a function can be called. A simple way to implement a debounce function is to use setTimeout() to call the function after a certain amount of time has passed.
  1. How can you implement a lazy loading in JavaScript ?
  • Lazy loading is a technique for loading content only when it is needed. This can be implemented by using IntersectionObserver which is a native JavaScript API and loading the content when the element is in the viewport.
  1. How would you explain the difference between call, apply, and bind?
  • call() and apply() are used to invoke a function with a specific this value and arguments, while bind() returns a new function with the this value and arguments bound to it.
  1. How would you explain the difference between let and const?
  • let is used for variables whose values will be reassigned and it is block scoped , const is used for variables that are assigned to a value once and cannot be reassigned, it is also block scoped.
  1. What is a pure function in javascript and why are they important?
  • A pure function is a function that does not depend on and does not modify the state of variables outside of its scope and always returns the same output for the same input. Pure functions are important because they are predictable, easy to test and maintain and also helps with functional programing.
  1. What is the use of arrow function in javascript and how it differs from regular functions?
  • An arrow function is a shorthand syntax for creating a function in JavaScript. It uses the => syntax instead of the function keyword. Arrow functions do not have their own this keyword, they inherit the this value of the scope in which they are defined. This makes them useful for working with this in callback functions, and also it’s a good practice for functional programming.

These are just a few examples of the types of questions you might be asked in a JavaScript interview. Of course, the specific questions will depend on the job and the company you are interviewing with.