Basic_QA.md

var vs let

Feature	var	let
Scope	Function Scoped	Block Scoped
Hoisting	Variables are hoisted and initialized to undefined.	Variables are hoisted but not initialized
Re-Declaration	Can be redeclared within the same scope	Can't be redeclared
Temporal Dead Zone	Doesn't have a Temporal Dead Zone	Has a temporal Dead Zone

Event Loop

• JS is single-threaded but uses event loop to handle async tasks.
• Queue (callbacks, promises) → Event Loop → Call Stack.
• Ensures non-blocking execution.

== vs ===

• == → Loose equality (performs type coercion).
• === → Strict equality (no coercion).

"5" == 5  // true
"5" === 5 // false

Closures

Function remembers variables from its lexical scope even after outer function execution ends.

Example

function outer() {
  let count = 0;
  return function inner() {
    return ++count;
  }
}
const counter = outer();
counter(); // 1
counter(); // 2

Hoisting

• Variables (var) and functions are hoisted to the top of their scope.
• var hoisted with undefined, let/const hoisted but in temporal dead zone.

Synch vs Async JS

• Synchronous → Executes line by line, blocking.
• Asynchronous → Doesn’t block, uses callbacks, promises, async/await.

Promises vs async/await

• Promise → Handles async results (then/catch).
• async/await→ Cleaner syntax, built on top of Promises.

async function getData() {
  const data = await fetch(...);
}

map(), forEach(), filter(), reduce()

• map → Transforms array, returns new array.
• forEach → Iterates, no return value.
• filter → Returns subset matching condition.
• reduce → Accumulates to a single value.

Debounce vs Throttle

• Debounce → Delay execution until after a pause in events.
• Throttle → Execute at most once in a given interval.

this keyword

• In object → Refers to object.
• In function → undefined in strict mode, else global object.
• In arrow function → Lexically bound (inherits from parent).

const obj = {
  name: "JS",
  normal: function() { console.log(this.name); },
  arrow: () => console.log(this.name)
};
obj.normal(); // "JS"
obj.arrow();  // undefined