Most Important JavaScript Interview Questions

Based on couple of Interviews,Here are the list of most important Question of JavaScript –

  • What are JavaScript Data Types?

Ans :  JavaScript provides different data types to hold different types of values. There are two types of data types in JavaScript.                                                        1- Primitive data type                         2- Non-primitive (reference) data type

1- JavaScript primitive data types

There are five types of primitive data types in JavaScript. They are as follows:

Data Type Description
String represents sequence of characters e.g. “upsilonwin”
Number represents numeric values e.g. 10
Boolean represents boolean value either false or true
Undefined represents undefined value
Null represents null i.e. no value at all

Note –  Symbol (new in ECMAScript 6, is treated as datatype )

2- JavaScript non-primitive data types

The non-primitive data types are as follows:

Data Type Description
Object represents instance through which we can access members
Array represents group of similar values
RegExp represents regular expression
  • How to create objects in JavaScript?
  • What are undeclared and undefined variables?
  • What is called Variable typing in Javascript?
  • What is the instanceof operator in JavaScript?
  • What is scope in JavaScript?
  • What does a JavaScript function result when it has no return statement?
  • What Is Prototype In JavaScript?
  • Explain the difference between “==” and “===”?
  • What is the function of delete operator?
  • What is an undefined value in JavaScript?
  • What is the difference between .call() and .apply()?
  • What is a “closure” in JavaScript
  • What is the callback approach in JavaScript?
  • What is hoisting in JavaScript?
  • What is lexical scope?
  • What is difference between let, var and const ?
  • Does JavaScript pass parameter by value or by reference?
  • Describe the properties of an anonymous function in JavaScript?
  • What is Immediately-Invoked Function Expression (IIFE) in JavaScript ?
  • How to create Singleton Object in Javascript? How will you verify its Singleton?
  • How Promises Work in Javascript?
  • What is the ‘Execution Context’ in JavaScript?


Happy Learning 🙂

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