Scope

2 min read | JavaScript Tutorial

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Global Scope

Variables declared outside any function or block are in the global scope and accessible everywhere.

const globalVar = "I am global";

function showGlobal() {
  console.log(globalVar); // accessible here
}

showGlobal(); // I am global
console.log(globalVar);  // I am global

Function Scope

Variables declared with var inside a function are scoped to that function and not visible outside.

function myFunction() {
  var secret = "hidden";
  console.log(secret); // "hidden"
}

myFunction();
// console.log(secret); // ReferenceError

Block Scope with let and const

Both let and const are block-scoped, meaning they exist only within the nearest set of curly braces.

if (true) {
  let blockLet = "I exist only here";
  const blockConst = "Me too";
  console.log(blockLet); // works
}
// console.log(blockLet);  // ReferenceError
// console.log(blockConst); // ReferenceError

Scope Chain

JavaScript looks up variables through nested scopes, from innermost to outermost.

const outer = "outer";

function middle() {
  const inner = "inner";
  function deep() {
    const deepest = "deepest";
    console.log(deepest); // deepest
    console.log(inner);   // inner (from parent)
    console.log(outer);   // outer (from grandparent)
  }
  deep();
}

middle();

var vs let vs const

var is function-scoped and hoisted. let and const are block-scoped and not initialized until declared (temporal dead zone).

function scopeDemo() {
  if (true) {
    var varX = "I leak out";
    let letY = "I stay inside";
  }
  console.log(varX); // "I leak out"
  // console.log(letY); // ReferenceError
}

scopeDemo();

Concepts Explained