Variables in JavaScript

Bindings control scope and mutability. Prefer const by default, let when reassignment is needed; avoid var.

let vs const vs var

let x = 1        // block-scoped, reassignable
const y = 2      // block-scoped, not reassignable (but object contents can mutate)
var z = 3        // function-scoped, hoisted; avoid in modern code

Hoisting and TDZ

let/const are hoisted but not initialized until their declaration (temporal dead zone). var initializes to undefined at function start.

console.log(a)    // ReferenceError (TDZ)
let a = 1

Function declarations are hoisted fully within their scope.

foo() // ok
function foo() {}

Reassignment vs mutation

const prevents rebinding, not mutation of object contents.

const obj = { n: 1 }
obj.n = 2         // ok
// obj = {}      // TypeError

Freeze to avoid mutation:

Object.freeze(obj)

Destructuring and defaults

Unpack arrays/objects with defaults and renaming; use rest/spread for copies and merges.

const [first, ...rest] = [1,2,3]
const { name: n = 'anon', age } = { age: 42 }
const copy = { ...obj, age: 43 }

Scope and blocks

Block scope for let/const; function scope for var; globalThis holds globals across environments.

if (true) {
  let hidden = 1
}
// hidden is not defined

Globals and strict mode

Avoid accidental globals—assigning to an undeclared identifier creates a global in sloppy mode (not in strict/ESM). Use strict mode/ESM.

Summary

• Default to const, use let when needed, avoid var
• Understand TDZ/hoisting and destructuring; minimize global usage