Top 5 JavaScript Interview Questions I Wish I Knew

Q1: Explain equality in JavaScript

JavaScript has both strict and type–converting comparisons:

• Strict comparison (e.g., ===) checks for value equality without allowing coercion
• Abstract comparison (e.g. ==) checks for value equality with coercion allowed

let a = '42';
let b = 42;
a == b; // true
a === b; // false

Some simple equalityrules:

If either value (aka side) in a comparison could be the true or false value, avoid == and use ===. If either value in a comparison could be of these specific values (0, "", or [] — empty array), avoid == and use ===. In all other cases, you’re safe to use ==. Not only is it safe, but in many cases it simplifies your code in a way that improves readability.

Q2: What does “use strict” do?

The use strict literal is entered at the top of a JavaScript program or at the top of a function and it helps you write safer JavaScript code by throwing an error if a global variable is created by mistake. For example, the following program will throw an error:

function foo(arg) {
  "use strict";
  x = arg + 10;
}`

It will throw an error because x was not defined and it is being set to some value in the global scope, which isn’t allowed with use strict The small change below fixes the error being thrown:

function foo(arg) {
  'use strict';
  var x = arg + 10;
}

Q3: Explain Null and Undefined in JavaScript

JavaScript (and by extension TypeScript) has two bottom types: null and undefined. They are intended to mean different things:

Something hasn’t been initialized: undefined. Something is currently unavailable: null.

Q4: Explain Values and Types in JavaScript

JavaScript has typed values, not typed variables. The following built-in types are available:

• string
• number
• boolean
• null and undefined
• object
• symbol (new to ES6)

Q5: What is let keyword in JavaScript?

In addition to creating declarations for variables at the function level, ES6 lets you declare variables to belong to individual blocks (pairs of { .. }), using the let keyword.