Scope: global, local and block

Scope is "who can see which variable"

Picture a house. What you leave in your bedroom can't be seen from the kitchen. Variables work the same way: each one lives in a "room" and is only visible from inside. We call that room the scope.

Getting this saves you from the classic "but I declared that variable, why does it say it doesn't exist?". Spoiler: it does exist, just in another room.

Global scope

A variable declared outside of any function or block is global: it's visible from anywhere in the program. Convenient, but risky if you overdo it: anyone can touch it.

const planet = "Earth"; // global
function show() {
  console.log(planet); // accessible here
}

Local scope (function)

Variables declared inside a function only exist there:

function compute() {
  const internal = 42;
  return internal;
}
// console.log(internal); // ❌ Error: internal doesn't exist outside

Block scope

With let and const, every pair of braces { } (an if, a for...) creates its own mini-room:

if (true) {
  let message = "hi";
  console.log(message); // ✅ "hi"
}
// console.log(message); // ❌ Error: outside the block it doesn't exist

⚠️ Classic trap: var does not respect block scope, only function scope. A var declared inside an if leaks outside the block and gives you surprises. That's why today we use let and const: they respect the room.

Examples

The global is visible from inside; the local stays locked in its function

const global = "I'm global";
function demo() {
  const local = "I'm local";
  console.log(global); // accessible
  console.log(local);  // accessible
}
demo();