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Complete JavaScript quick revision notes covering all important concepts for interviews and exams. Rapid-fire key concepts with code examples and outputs for last-minute preparation.
Last-minute revision guide covering ALL important JavaScript concepts. Short explanations + code + output. Perfect for interviews & exams.
2. Hoisting
JS moves declarations to the top of their scope during the memory phase. But behavior differs:
| Declaration | Hoisted? | Initial Value | TDZ? |
|---|---|---|---|
var | ✅ | undefined | No |
let / const | ✅ | ❌ Not initialized | Yes |
function declaration | ✅ | Full function body | No |
function expression | ✅ | undefined (var) or TDZ (let) | Depends |
javascript example
console.log(x); // undefined (var hoisted)
console.log(y); // ReferenceError: Cannot access 'y' before initialization
var x = 10;
let y = 20;
greet(); // "Hello!" (function declaration hoisted fully)
function greet() { console.log("Hello!"); }Output
undefined ReferenceError: Cannot access 'y' before initialization
3. Closures
A closure is a function that remembers variables from its outer (lexical) scope even after the outer function has returned. In simple terms: Closure means the inner function remembers the outer scope.
Example
function counter() {
let count = 0;
return function increment() {
count++;
console.log(count);
};
}
const add = counter();
add(); // 1
add(); // 2
add(); // 3
Output
1 2 3
Use cases: Data privacy, memoization, function factories, setTimeout callbacks.
4. this Keyword — 4 Rules
| Rule | Context | this points to |
|---|---|---|
| 1. Default Binding | Standalone function call | window (non-strict) / undefined (strict) |
| 2. Implicit Binding | Method call obj.fn() | The object before the dot |
| 3. Explicit Binding | call/apply/bind | Whatever you pass |
4. new Binding | new Constructor() | Newly created object |
Arrow functions: Don't have their own this. They inherit from parent lexical scope.
Example
const obj = {
name: "WoHo",
regular: function() { console.log(this.name); },
arrow: () => { console.log(this.name); }
};
obj.regular(); // "WoHo" (implicit binding)
obj.arrow(); // undefined (arrow inherits global this)
Output
WoHo undefined
5. Prototypes & Inheritance
Every JS object has a hidden [[Prototype]] link (accessible via __proto__). When you access a property not on the object, JS walks up the prototype chain.
javascript example
function Person(name) {
this.name = name;
}
Person.prototype.greet = function() {
return `Hi, I'm ${this.name}`;
};
const p = new Person("Rahul");
console.log(p.greet()); // "Hi, I'm Rahul"
console.log(p.__proto__ === Person.prototype); // true
console.log(Person.prototype.__proto__ === Object.prototype); // trueOutput
Hi, I'm Rahul true true
Chain: p → Person.prototype → Object.prototype → null
6. Event Loop
JS is single-threaded but non-blocking thanks to the Event Loop.
┌─────────────────────────────────┐
│ CALL STACK │ ← Executes sync code
└───────────────┬─────────────────┘
│ (empty?)
▼
┌─────────────────────────────────┐
│ MICROTASK QUEUE │ ← Promises (.then), queueMicrotask
│ (Higher priority, drains first)│
└───────────────┬─────────────────┘
│ (empty?)
▼
┌─────────────────────────────────┐
│ MACROTASK QUEUE │ ← setTimeout, setInterval, I/O
│ (One task per loop tick) │
└─────────────────────────────────┘
Order: Sync code → All Microtasks → One Macrotask → All Microtasks → repeat
Example
console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");
Output
1 4 3 2
Explanation: Sync first (1, 4) → Microtask (3) → Macrotask (2)
7. Promises & Async/Await
A Promise represents a future value. States: pending → fulfilled / rejected.
Example
// Promise creation
const fetchData = () => new Promise((resolve, reject) => {
setTimeout(() => resolve("Data loaded!"), 1000);
});
// Using .then
fetchData().then(data => console.log(data));
// Using async/await (cleaner syntax, same thing)
async function getData() {
try {
const data = await fetchData();
console.log(data);
} catch (err) {
console.error(err);
}
}
getData();
Output
Data loaded! Data loaded!
Key methods: Promise.all() (all must resolve), Promise.race() (first to settle), Promise.allSettled() (wait for all, no short-circuit), Promise.any() (first to resolve).
8. Scope — Global, Function, Block
| Scope | var | let/const |
|---|---|---|
| Global | ✅ | ✅ (but not on window) |
| Function | ✅ Confined | ✅ Confined |
Block {} | ❌ Leaks out | ✅ Confined |
javascript example
if (true) {
var a = 10;
let b = 20;
const c = 30;
}
console.log(a); // 10 (var leaks out of block)
console.log(b); // ReferenceError (let is block-scoped)Output
10 ReferenceError: b is not defined
Scope Chain: Inner scope → Outer scope → ... → Global scope (lexical/static).
9. == vs ===
==(loose equality): Performs type coercion before comparison.===(strict equality): No coercion, both type and value must match.
javascript example
console.log(0 == ""); // true (both coerce to 0)
console.log(0 === ""); // false (number vs string)
console.log(null == undefined); // true (special rule)
console.log(null === undefined); // false
console.log(NaN == NaN); // false (NaN is never equal to anything)
console.log(1 == "1"); // true
console.log(1 === "1"); // falseOutput
true false true false false true false
Rule: Always use === unless you intentionally want coercion.
10. Spread / Rest / Destructuring
Spread (...) — Expands elements
Example
const arr1 = [1, 2];
const arr2 = [...arr1, 3, 4]; // [1, 2, 3, 4]
const obj1 = { a: 1 };
const obj2 = { ...obj1, b: 2 }; // { a: 1, b: 2 }
Rest (...) — Collects remaining elements
Example
function sum(...nums) {
return nums.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3, 4)); // 10
Output
10
Destructuring — Extract values cleanly
Example
const [x, y, ...rest] = [10, 20, 30, 40];
console.log(x, y, rest); // 10 20 [30, 40]
const { name, age = 25 } = { name: "Amit" };
console.log(name, age); // "Amit" 25
Output
10 20 [30, 40] Amit 25
11. map vs filter vs reduce
Example
const nums = [1, 2, 3, 4, 5];
// map — transform each element, returns new array
const doubled = nums.map(n => n * 2); // [2, 4, 6, 8, 10]
// filter — keep elements that pass test
const evens = nums.filter(n => n % 2 === 0); // [2, 4]
// reduce — accumulate into single value
const total = nums.reduce((acc, n) => acc + n, 0); // 15
Output
doubled: [2, 4, 6, 8, 10] evens: [2, 4] total: 15
Remember: map transforms, filter selects, reduce accumulates.
12. Event Bubbling & Delegation
Bubbling: Event triggers on target, then bubbles UP through ancestors (child → parent → grandparent).
Capturing: Event goes DOWN from root to target (rarely used).
Event Delegation: Attach ONE listener on parent, handle events from children via event.target. Great for dynamic lists!
Example
// Instead of adding listener to each <li>:
document.querySelector("ul").addEventListener("click", (e) => {
if (e.target.tagName === "LI") {
console.log("Clicked:", e.target.textContent);
}
});
Stop bubbling: e.stopPropagation() | Prevent default: e.preventDefault()
13. localStorage vs sessionStorage
| Feature | localStorage | sessionStorage |
|---|---|---|
| Lifetime | Until manually cleared | Until tab/window closes |
| Scope | Same origin (all tabs) | Same origin + same tab |
| Size | ~5-10 MB | ~5-10 MB |
| Survives refresh? | ✅ Yes | ✅ Yes |
| Survives tab close? | ✅ Yes | ❌ No |
javascript example
// Store
localStorage.setItem("user", JSON.stringify({ name: "Priya" }));
// Retrieve
const user = JSON.parse(localStorage.getItem("user"));
console.log(user.name); // "Priya"
// Remove
localStorage.removeItem("user");
// Clear all
localStorage.clear();Output
Priya
14. call / apply / bind
All three are used for explicit binding of this.
Example
const person = { name: "Sita" };
function greet(city, country) {
console.log(`${this.name} from ${city}, ${country}`);
}
// call — invokes immediately, args comma-separated
greet.call(person, "Delhi", "India");
// apply — invokes immediately, args as array
greet.apply(person, ["Delhi", "India"]);
// bind — returns NEW function, does NOT invoke immediately
const boundGreet = greet.bind(person, "Delhi", "India");
boundGreet();
Output
Sita from Delhi, India Sita from Delhi, India Sita from Delhi, India
Mnemonic: Call = Comma, Apply = Array, Bind = returns Bound function.
15. Debounce vs Throttle
Both limit function execution frequency — crucial for performance optimization.
Debounce — Wait until user STOPS, then fire once
Use case: Search input, window resize. Fires after a pause.
Example
function debounce(fn, delay) {
let timer;
return function(...args) {
clearTimeout(timer);
timer = setTimeout(() => fn.apply(this, args), delay);
};
}
const search = debounce((q) => console.log("Searching:", q), 300);
// Only last call fires after 300ms of no input
Throttle — Fire at most once every X ms (regular intervals)
Use case: Scroll events, button clicks. Ensures steady rate.
Example
function throttle(fn, limit) {
let inThrottle = false;
return function(...args) {
if (!inThrottle) {
fn.apply(this, args);
inThrottle = true;
setTimeout(() => inThrottle = false, limit);
}
};
}
const handleScroll = throttle(() => console.log("Scrolled!"), 1000);
// Fires at most once per second during scrolling
Memory Trick: Debounce = "Wait, let everything finish first" | Throttle = "Once every 1 second"
16. 🧠 Common Tricky Output Questions
Q1: var in loop with setTimeout
Example
for (var i = 0; i < 3; i++) {
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