JavaScript flashcards

10 cards. Answer in your head, flip, and be honest with yourself.

0Still learning1 / 10Know0

var vs let vs const

Scope

js
function example() {
  var a = 1;   // function-scoped
  let b = 2;   // block-scoped
  const c = 3; // block-scoped

  if (true) {
    var a = 10;   // same variable - overwrites!
    let b = 20;   // new variable - only inside this block
    console.log(b); // 20
  }

  console.log(a); // 10 - was overwritten
  console.log(b); // 2 - untouched
}

Hoisting

js
console.log(a); // undefined - hoisted but not initialized
console.log(b); // ReferenceError - temporal dead zone

var a = 1;
let b = 2;

Reassignment

js
var x = 1;   x = 2;     // ok
let y = 1;   y = 2;     // ok
const z = 1; z = 2;     // Error - cannot reassign

// const with objects - reference is locked, contents are not
const user = { name: 'Alice' };
user.name = 'Bob'; // ok
user = {};         // Error

Comparison

	`var`	`let`	`const`
Scope	Function	Block	Block
Hoisting	Yes (undefined)	TDZ	TDZ
Reassign	Yes	Yes	No
Re-declare	Yes	No	No

Rule: use const by default, let when you need to reassign, never use var.

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