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JavaScript Handbook

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Beginner Level JavaScript Interview Questions and Answers

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Beginner Level JavaScript Interview Questions and Answers

1. What is JavaScript, and what are its key features?

Answer: JavaScript is a high-level, interpreted programming language primarily used to add interactivity to web pages. It can run client-side in browsers or server-side via Node.js.

Key Features:

  • Dynamic and weakly typed
  • Supports object-oriented, functional, and imperative programming
  • Event-driven and asynchronous (callbacks, promises, async/await)
  • Runs in browsers and other environments

Example:

document.getElementById("myButton").addEventListener("click", () => alert("Clicked!"));

2. How do you set up a JavaScript development environment?

Answer:

  • A text editor (e.g., VS Code)
  • A browser (e.g., Chrome) with developer tools
  • Optionally, Node.js for server-side JavaScript
  • Include JS using <script> tags or external .js files

Example:

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

3. Where can JavaScript be embedded in an HTML file?

Answer:

  • <script> tags in <head> or <body>
  • External files via <script src="file.js"></script>
  • Inline scripts: <button onclick="myFunction()">Click</button>

Example:

<script>
  console.log("Hello, World!");
</script>

4. What are the different ways to output data in JavaScript?

Answer:

  • console.log() → Console output
  • alert() → Popup message
  • document.write() → Writes directly to the document (not recommended for dynamic content)

Example:

console.log("Log to console");
alert("This is an alert");
document.write("Written to document");

5. What is the basic syntax of JavaScript?

Answer:

  • Case-sensitive
  • Semicolons to end statements (optional with ASI)
  • Curly braces {} for code blocks
  • Parentheses () for function calls and conditions

Example:

let name = "John";
if (name) {
  console.log("Hello, " + name);
}

6. What are JavaScript statements?

Answer: Instructions executed by the browser. Examples include variable declarations, loops, and conditionals.

Example:

let x = 5; // Declaration
x = x + 1; // Assignment

7. What are JavaScript comments, and why are they used?

Answer: Comments explain code or prevent execution.

  • Single-line: // Comment
  • Multi-line: /* Comment */

Example:

// Single-line comment
/* Multi-line
   comment */

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

Answer:

  • var: Function-scoped, can be redeclared, hoisted
  • let: Block-scoped, can be updated, not redeclared in same scope
  • const: Block-scoped, cannot be updated, must be initialized

Example:

var a = 1;
let b = 2;
const c = 3;

9. What is variable scope in JavaScript?

Answer: Determines where a variable is accessible:

  • Global: Accessible everywhere
  • Local: Accessible inside functions
  • Block: let and const limited to {}

Example:

let globalVar = "I'm global";
function myFunc() {
  let localVar = "I'm local";
  console.log(globalVar); // Accessible
  console.log(localVar); // Accessible
}

10. What is hoisting in JavaScript?

Answer: Moving declarations to the top of their scope. Only declarations, not initializations, are hoisted.

Example:

console.log(x); // undefined
var x = 5;

11. What are arithmetic operators in JavaScript?

Answer: +, -, *, /, %, **, ++, --

Example:

let a = 10, b = 5;
console.log(a + b); // 15
console.log(a ** 2); // 100

12. What are comparison operators?

Answer: ==, ===, !=, !==, >, <, >=, <=

Example:

console.log(5 == "5");  // true
console.log(5 === "5"); // false

13. What is the purpose of logical operators?

Answer: Combine boolean expressions: &&, ||, !

Example:

let a = true, b = false;
console.log(a && b); // false
console.log(a || b); // true

14. What are primitive data types in JavaScript?

Answer: String, Number, BigInt, Boolean, Null, Undefined, Symbol

Example:

let str = "Hello";
let num = 42;
let bool = true;

15. What is the difference between null and undefined?

Answer:

  • null: Intentional absence of value
  • undefined: Variable declared but not assigned

Example:

let x = null;
let y;

16. How does the typeof operator work?

Answer: Returns the type of a variable as a string

Example:

console.log(typeof 42);   // "number"
console.log(typeof null); // "object"

17. What is type conversion in JavaScript?

Answer: Changing a value’s type, either explicitly or implicitly

Example:

let num = "123";
console.log(Number(num) + 1); // 124 (explicit)
console.log(num + 1);         // "1231" (implicit)

18. What are string methods in JavaScript?

Answer: Methods like toUpperCase(), toLowerCase(), slice(), trim(), replace()

Example:

let str = "Hello World";
console.log(str.toUpperCase()); // "HELLO WORLD"
console.log(str.slice(0, 5));   // "Hello"

19. What are number methods in JavaScript?

Answer: toFixed(), toPrecision(), parseInt(), parseFloat()

Example:

let num = 3.14159;
console.log(num.toFixed(2)); // "3.14"

20. What is the Math object used for?

Answer: Provides constants and functions like Math.PI, Math.random(), Math.floor()

Example:

console.log(Math.random());     // Random between 0 and 1
console.log(Math.floor(3.7));  // 3

21. What are booleans in JavaScript?

Answer: Represent true/false values, often used in conditionals

Example:

let isAdult = true;
if (isAdult) console.log("Eligible"); // "Eligible"

22. What is an array in JavaScript?

Answer: An ordered list of values indexed from 0

Example:

let arr = [1, 2, 3];
console.log(arr[0]); // 1

23. What are common array methods?

Answer: push(), pop(), shift(), unshift(), slice(), splice()

Example:

let arr = [1, 2];
arr.push(3);       // [1,2,3]
console.log(arr.pop()); // 3

24. How does the if statement work in JavaScript?

Answer: Executes a block of code if a condition is true, can use else/else if

Example:

let age = 18;
if (age >= 18) {
  console.log("Adult");
} else {
  console.log("Minor");
}

25. What is the purpose of a for loop?

Answer: Repeats a block of code a specified number of times

Example:

for (let i = 0; i < 3; i++) {
  console.log(i); // 0, 1, 2
}

26. Difference between break and continue in loops

Answer:

  • break: exits loop entirely
  • continue: skips current iteration

Example:

for (let i = 0; i < 5; i++) {
  if (i === 3) break;
  console.log(i); // 0,1,2
}

27. What is block scope in JavaScript?

Answer: Variables declared with let or const are limited to the {} block

Example:

{
  let x = 10;
  console.log(x); // 10
}
console.log(x); // ReferenceError

28. What is a function declaration in JavaScript?

Answer: Named function using function keyword, hoisted to the top

Example:

function greet() {
  return "Hello!";
}
console.log(greet()); // "Hello!"

29. Difference between parameters and arguments

Answer:

  • Parameters: Variables in function definition
  • Arguments: Values passed when calling the function

Example:

function add(a, b) { // parameters
  return a + b;
}
console.log(add(2, 3)); // 2,3 are arguments

30. What is an object in JavaScript?

Answer: Collection of key-value pairs

Example:

let person = { name: "John", age: 30 };
console.log(person.name); // "John"

 

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