JavaScript Exam Questions

Most-asked JavaScript exam questions for BCA, B.Tech, and campus interviews.

• What is JavaScript? A high-level, interpreted, multi-paradigm scripting language for web pages. Created by Brendan Eich in 1995.
• Where does JS run? In any JavaScript engine — V8 (Chrome, Node.js), SpiderMonkey (Firefox), JavaScriptCore (Safari).
• Is JS compiled or interpreted? Modern engines do JIT compilation (just-in-time) — compile hot code to machine code on the fly.
• var, let, const?
  • var — function-scoped, hoisted, can be re-declared.
  • let — block-scoped, NOT hoisted to top.
  • const — block-scoped, immutable binding (object contents are still mutable).
• == vs ===? == does type coercion (5 == '5' → true). === strict (no coercion).

• WAP to find sum of digits of a number.
• WAP to reverse a string without reverse().
• WAP to find max in array using loop.
• WAP using addEventListener to alert on button click.
• WAP to validate a form (empty fields, email pattern).

js Run in compiler
console.log(typeof null);          // 'object'
console.log(typeof undefined);     // 'undefined'
console.log([] + []);              // '' (empty string)
console.log([] + {});              // '[object Object]'
console.log(0.1 + 0.2 === 0.3);    // false (floating-point)

• let / const
• Arrow functions: (a, b) => a + b
• Template literals: `Hello ${name}`
• Destructuring: const { a, b } = obj;
• Spread/rest: [...arr], (...args) => …
• Promises + async/await
• Classes: class Foo { constructor() {} }
• Modules: import x from 'y'

• DOM = Document Object Model — tree of HTML elements as JS objects.
• document.getElementById("x"), document.querySelector(".btn").
• Event listeners: el.addEventListener("click", fn).
• innerHTML (parses HTML) vs textContent (treats as text).

• Always use === in production — avoids coercion bugs.
• Default to const, fall back to let only when reassigning.
• Use async/await instead of nested .then() for readable async code.
• Mention event bubbling when discussing event handlers — high-mark answer.