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Block Scope

Block Scope

Introduction

Block scope confines variables to the block {} where they are declared. let and const have block scope, while var does not.

Visual Explanation

flowchart LR
    A["{ let x = 1; }"]
    B["x is accessible here"]
    C["x is NOT accessible outside"]

    A --> B
    A --> C
    C --> D["ReferenceError!"]

let and const are Block Scoped

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

console.log(x); // ReferenceError
console.log(y); // ReferenceError

var is NOT Block Scoped

{
  var z = 30;
}

console.log(z); // 30 — var leaks out of the block!

Practical Example

// Block scope prevents counter leaks
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Logs: 0, 1, 2 (each i is block-scoped)

// VS var — all share the same variable
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Logs: 3, 3, 3 (only one i)

Summary

let and const are block scoped
var is function scoped (not block scoped)
Block scope prevents variable leakage
Always prefer let/const over var

Further Reading

MDN: Block Statement