JavaScript Interview Questions

Welcome to our JavaScript interview questions repository! Here, you'll find a collection of common JavaScript interview questions along with their answers and examples.

Question 1: What is a closure in JavaScript?

Answer:

A closure is a function bundled together with its lexical environment (the scope chain) at the time of its creation. It allows the function to retain access to variables from its containing scope even after that scope has finished executing.

Example:

function outerFunction() {
  let outerVariable = "I am from the outer function";

  function innerFunction() {
    console.log(outerVariable);
  }

  return innerFunction;
}

const closureExample = outerFunction();
closureExample(); // Output: I am from the outer function 

Question 2: Explain the difference between var, let, and const in JavaScript.?

Answer:

var: Function-scoped (declared within a function, accessible within that function and any nested functions). Can be redeclared and reassigned within the same scope, leading to potential scoping issues. let: Block-scoped (declared within a block of code using curly braces {}, accessible only within that block). Cannot be redeclared within the same scope, prevents accidental overwrites. Can be reassigned within the same block. const: Block-scoped, similar to let. Cannot be redeclared or reassigned after initialization, ensures data integrity.

Example:

function example() {
  var x = 10; // Function-scoped, accessible within the function
  if (true) {
    let y = 20; // Block-scoped, accessible only within this block
    const z = 30; // Block-scoped, cannot be reassigned or redeclared
  }

  console.log(x); // Accessible: 10
  // console.log(y); // ReferenceError: y is not defined (outside its block)
  console.log(z); // Accessible: 30
}

example();

Question 3. What's the difference between undefined and not defined in JavaScript

Answer

In JavaScript if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error var name is not defined and the script will stop executing thereafter. But If you use typeof undeclared_variable then it will return undefined.

Before starting further discussion let's understand the difference between declaration and definition.

var x is a declaration because we are not defining what value it holds yet, but we are declaring its existence and the need for memory allocation.

var x; // declaring x console.log(x); // output: undefined

var x; // Declaration typeof x === 'undefined'; // Will return true

console.log(y); // Output: ReferenceError: y is not defined