Variables: let and const

JavaScript · Reference cheat sheet

📋 Overview

Modern JavaScript declares variables with let and const. Avoid var in new code. const prevents reassignment of the binding; let allows reassignment. Both are block-scoped.

🔧 Core concepts

const objects/arrays can still have their contents mutated — the binding cannot point to a new object.

💡 Examples

Basic declarations:

const name = "Sam";
let score = 0;

score = score + 10;
// name = "Alex"; // TypeError
console.log(name, score);

Block scope:

let x = 1;
{
  let x = 2;
  console.log(x); // 2
}
console.log(x); // 1

const with objects:

const user = { id: 1, role: "guest" };
user.role = "admin"; // OK — mutating property
// user = {};       // TypeError — rebinding

const nums = [1, 2];
nums.push(3); // OK
console.log(user, nums);

Temporal dead zone (use before declare):

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

⚠️ Pitfalls

• Prefer const until you know a value must change.
• const ≠ deeply immutable.
• Redeclaring the same let/const in one scope is a SyntaxError.
• Forgetting let/const creates an accidental global in non-strict sloppy mode.

🔗 Related

• hello_world.md
• typeof_basics.md
• null_undefined.md
• template_basics.md