What is the difference between let, const, and var?

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1. var

  • Scope → Function-scoped (available within the function where it’s declared).

  • Hoisting → Hoisted and initialized as undefined.

  • Redeclaration → Can be re-declared in the same scope.

  • Reassignment → Allowed.

var x = 10;
var x = 20;  // ✅ redeclared
x = 30;      // ✅ reassigned

2. let

  • Scope → Block-scoped (limited to { } where it’s defined).

  • Hoisting → Hoisted but kept in Temporal Dead Zone (TDZ) until initialized (cannot use before declaration).

  • Redeclaration → ❌ Not allowed in the same scope.

  • Reassignment → ✅ Allowed.

let y = 10;
// let y = 20;  ❌ redeclaration not allowed
y = 30;        // ✅ reassignment allowed

3. const

  • Scope → Block-scoped (like let).

  • Hoisting → Hoisted but in TDZ.

  • Redeclaration → ❌ Not allowed.

  • Reassignment → ❌ Not allowed. (value is constant)

  • However, for objects and arrays, the reference is constant, but properties/elements can still be changed.

const z = 10;
// z = 20;  ❌ Error (cannot reassign)

const obj = { name: "JS" };
obj.name = "JavaScript"; // ✅ allowed (modifying property)

✅ Quick Comparison Table

Featurevarletconst
ScopeFunction-scopedBlock-scopedBlock-scoped
HoistingYes (initialized as undefined)Yes (TDZ)Yes (TDZ)
Redeclaration✅ Allowed❌ Not allowed❌ Not allowed
Reassignment✅ Allowed✅ Allowed❌ Not allowed
Use CaseLegacy codeVariables that changeFixed values / references

👉 Best practice in modern JavaScript:

  • Use const by default.

  • Use let if you need reassignment.

  • Avoid var unless working with old code.

Read More:


What is hoisting in JavaScript?

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