JavaScript Interview Questions for Freshers: Essential Concepts Explained

When preparing for JavaScript interviews, it's crucial to grasp the basic concepts thoroughly. Below are some key questions often asked at the entry level, explained in a beginner-friendly way.

1) What is Hoisting in JavaScript?

Hoisting is a JavaScript mechanism where variables, functions, or classes are moved to the top of their scope before the code is executed. This means you can use a function or variable before declaring it in the code.

2) Explain Closures.

A closure is a function combined with its lexical environment. In simpler terms, closures allow a function to remember the variables from its outer scope even when the function is executed outside that scope.

3) What is the "debugger" Statement in JavaScript?

The debugger statement pauses the execution of JavaScript code and invokes any available debugging functionality. This allows developers to check the state of the code at that moment. However, it will not work if the browser’s debugging tool is not open.

4) What are First-Class Functions?

In JavaScript, first-class functions are treated as objects. This means functions can be stored in variables, passed as arguments to other functions, or returned from other functions—just like any other value.

5) What are Higher-Order Functions?

A higher-order function is one that accepts another function as an argument or returns a function as a result. This enables powerful patterns like callbacks, event handlers, and function composition.

6) Why Use the typeof Operator?

The typeof operator is used to check the type of a variable. It returns a string indicating the type of the value, such as 'string', 'number', 'boolean', etc.

7) What is the DOM?

The DOM (Document Object Model) is a programming interface for web documents. It represents the page so that programs can change the structure, style, and content of a document.

8) How Do You Create JavaScript Objects?

There are several ways to create objects in JavaScript:

• Using object literals: let obj = {};
• Using the new keyword: let obj = new Object();
• Using constructor functions
• Using Object.create()
• Using functions (factory methods)

9) What is WeakMap in JavaScript?

A WeakMap is a collection of key-value pairs where the keys are weakly referenced. This means that if there are no other references to the key object, it can be garbage collected, improving memory management.

10) What’s the Difference Between == and ===?

• == checks if two values are equal but performs type coercion (so '5' == 5 is true).
• === checks both the value and the type (so '5' === 5 is false).

11) What is Debouncing?

Debouncing is a technique to ensure that a function is only triggered after a specified pause in activity, such as when typing or scrolling. This can significantly improve performance by reducing unnecessary function calls.

12) How Do You Add JavaScript to an HTML File?

You can add JavaScript to an HTML file by including it in the <script> tag. For example:

html        

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<script src="app.js"></script>

13) What Are the Three Types of JavaScript Errors?

1. Syntax Errors: Mistakes in the code's syntax (e.g., missing brackets).
2. Runtime Errors: Errors that occur while the script is running.
3. Logical Errors: The code runs but does not produce the expected result due to a flaw in the logic.

14) What Are Async and Defer Attributes in Script Tags?

• Async: The script loads asynchronously and runs as soon as it’s ready, without waiting for the HTML to finish parsing.
• Defer: The script is executed after the HTML is fully parsed, maintaining the order of scripts.

15) Is JavaScript Dynamic or Static?

JavaScript is a dynamic language, meaning variables can change types at runtime.

16) What is the Difference Between null and undefined?

• null: Represents an intentional absence of a value.
• undefined: Indicates that a variable has been declared but hasn't been assigned a value.

17) What is Prototype Chaining?

Prototype chaining allows one object to inherit properties from another. If an object doesn't have a property directly, it can look up its prototype chain to find the property.

18) What is BOM?

The BOM (Browser Object Model) refers to all the objects provided by the browser to interact with the web page, like window, navigator, and location. It enables interaction with the browser environment outside of the web page.

19) What is the isNaN Function?

The isNaN() function checks whether a value is not a number. If the value is not a number, it returns true; otherwise, it returns false.

20) What is the Purpose of "use strict"?

"use strict" is a directive that enforces stricter parsing and error handling in your code. It prevents you from using undeclared variables, among other things, making your code safer and less error-prone.

This structured approach with clear explanations helps freshers get a solid understanding of fundamental JavaScript concepts, which can help in technical interviews.