🧠 JavaScript let Keyword – Block Scope & TDZ Explained

💡 Ever encountered weird bugs when using var in a loop or block?
That’s where let comes to the rescue.

The let keyword in JavaScript allows you to declare block-scoped variables—ensuring safer, more predictable behavior in modern JS code. With the introduction of ES6, let quickly became the preferred way to declare variables that change over time.

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📘 What You’ll Learn

By the end of this guide, you’ll understand:

• ✅ How let works in JavaScript with real-world examples
• ✅ The key differences between let and var
• ✅ The concept of the Temporal Dead Zone (TDZ)
• ✅ Best practices to avoid common bugs

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✅ let Syntax

let variableName = value;

🔍 Line-by-Line Explanation:

• let: The keyword used to declare the variable
• variableName: The name of your variable
• = value: Assigns a value to the variable (optional at declaration)

📌 Example:

let age = 25;
console.log(age); // Output: 25

• let age = 25; — Declares a block-scoped variable age and assigns it 25.
• console.log(age); — Outputs the value 25 to the console.

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🧩 Features of let

🔹 1. Block Scope

Variables declared with let are only accessible within the block ({}) they’re declared in.

{
  let x = 10;
  console.log(x); // 10 ✅ accessible here
}
console.log(x); // ❌ ReferenceError: x is not defined

💡 Block scope prevents variables from leaking into other scopes—reducing side effects.

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🔹 2. No Redeclaration in Same Scope

You cannot declare the same variable twice in the same block with let.

let name = "Alice";
// let name = "Bob"; ❌ SyntaxError: Identifier 'name' has already been declared

🛑 This avoids accidental redeclarations—a common source of bugs with var.

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🔹 3. Reassignable

Unlike const, you can reassign values to a let variable.

let score = 90;
score = 95; // ✅ Allowed
console.log(score); // 95

📌 let is ideal when the variable value will change later in the program.

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🔹 4. Temporal Dead Zone (TDZ)

Variables declared with let exist in a “temporal dead zone” from the start of the block until the declaration is processed.

console.log(a); // ❌ ReferenceError
let a = 5;

💡 Even though a is hoisted, it is not initialized—accessing it before declaration triggers an error.

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🔄 let vs var – Comparison Table

Feature	let	var
Scope	Block-scoped	Function-scoped
Redeclaration	❌ Not allowed	✅ Allowed
Hoisting	Hoisted (TDZ applies)	Hoisted (initialized undefined)
Default Value	Not initialized	undefined
Use in Loops	Safe (per-iteration scope)	Risky (shared scope)

✅ Use let for safer scoping and cleaner code.

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🧪 Practical Example – Loop Closure with let vs var

✅ Using let (Correct output):

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1000); // 0, 1, 2
}

Explanation:

• Each iteration creates a new i bound to that specific loop.
• setTimeout correctly captures the value.

❌ Using var (Incorrect output):

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1000); // 3, 3, 3
}

Why this happens:

• var is function-scoped, so all timeouts share the same i.
• After the loop ends, i becomes 3, and all functions reference that.

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✅ Best Practices with let

✔️ Use let when:

• The variable needs to be reassigned (e.g., loop counters, flags)
• You want to limit scope to a block or {} to avoid leakage
• You want to prevent redeclarations within the same block

❌ Avoid let when:

• You need a constant value → use const
• You are unaware of scope rules → it might behave unexpectedly

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❓ Frequently Asked Questions (FAQs)

❓ Can I use let without assigning a value?

✅ Yes. You can declare and initialize later:

let user;
user = "John";
console.log(user); // John

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❓ What happens if I access a let variable before it’s declared?

⛔ You’ll get a ReferenceError due to TDZ:

console.log(a); // ❌ ReferenceError
let a = 10;

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❓ Is let hoisted?

Yes, but it’s not initialized until the line is reached. The time between block start and declaration is the Temporal Dead Zone.

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❓ Can let be used globally?

Yes, but it’s scoped to the script/module, not added to the global window object like var.

let x = 100;
console.log(window.x); // undefined

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📌 Summary

• let provides block-level scoping, protecting variables from unintentional usage across blocks.
• Unlike var, it prevents redeclaration and avoids closure bugs in loops.
• It introduces the concept of Temporal Dead Zone (TDZ) for safer coding practices.

👉 Mastering let is essential for writing modern, bug-free JavaScript.

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