Top Javascript Interview Questions and Answers (2024)

JavaScript is a web programming language. It is one of the web's core languages, along with HTML and CSS. While HTML defines the content and structure of the page, CSS defines how the elements are displayed, and JavaScript defines how the elements on the webpage behave on the web and facilitates user interaction.

Javascript is a must-have skill for anyone wanting a web development career. Here are some JavaScript Interview questions and answers that any fresher or experienced developer should know when attending an interview. 

1. What is the difference between let, var and const?

• The let keyword was introduced in ES6
• let cannot be redeclared, whereas var can be declared
• With Let, you cannot do this:

                 let name = "Ashok";
                 let name = 0;

• Whereas using var, you can do like this:

                 var name = "Ashok";
                 var name = 0;

• But redeclaring a variable inside a block can also impose problems
• For Eg,

     var name = "Ashok"; // The value of name is Ashok
     {
         var name = "Biju" // The value of name is Ashok
     }

     // The value of name is Biju

• let and const can not be redeclared. Whereas var can be redeclared
• Let is used in loops and conditionals.

2. What are the new features of Javascript ES6?

The new features in Javascript enhance code readability, maintainability and performance.

Here are the new features of Javascript ES6:

• Arrow Functions: Concise syntax for writing functions.
• Promises: Simplify asynchronous programming.
• Template Literals: Dynamic strings using backticks 
• let & const: Block-scoped variable declarations for better control.
• Default Parameters: Set default values in functions.
• Spread & Rest Operators: Expand or collect elements with `...`.
• Destructuring: Extract values from arrays or objects easily.
• Classes: Cleaner syntax for creating objects and inheritance.
• Enhanced Object Literals: Shorthand properties and methods.
• Modules: Import/export functionality between files.

3. List some string methods in Javascript

• Length
• Slice
• Replace
• Upper and Lower Case
• Contact
• Trim
• Char At
• Split

4. How do you shorthand this code?

     let newName;
     if( name ){
         newName = name;
     }else{
         newName = 'Default';
     }

Solution:

           const newName = name || 'Default';