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let/const Hoisting

let/const Hoisting

Introduction

let and const are hoisted (they do exist in memory during the creation phase), but they are NOT initialized until the actual declaration is reached. This period between scope entry and declaration is called the Temporal Dead Zone (TDZ).

The Key Insight

// ❌ ReferenceError: Cannot access before initialization
console.log(myLet);
let myLet = 10;

// myLet IS hoisted — it exists in memory
// BUT it's in the TDZ — accessing it throws an error
// This is different from var (which returns undefined)

Block Scope

let and const are block-scoped — they only exist within the nearest {}:

if (true) {
  let x = 10;
  const y = 20;
  console.log(x); // 10
  console.log(y); // 20
}
console.log(x); // ReferenceError
console.log(y); // ReferenceError

TDZ in Practice

// TDZ exists from scope start to declaration
{
  // TDZ starts here for x
  // console.log(x); — would throw ReferenceError
  let x = 10; // TDZ ends here
  console.log(x); // ✅ 10 — after TDZ
}

// typeof is also affected
{
  // console.log(typeof x); // ReferenceError!
  let x = 10;
}

const Must Be Initialized

let a; // ✅ OK — undefined
a = 10;

const b; // ❌ SyntaxError: Missing initializer
const c = 10; // ✅ OK

Summary

Aspect	let	const
Hoisted	✅ Yes	✅ Yes
Initialized	❌ No (TDZ)	❌ No (TDZ)
Scope	Block	Block
Reassignment	✅	❌
Must initialize	❌	✅

Further Reading