Scope

Syntax

{ /* block scope */ }
function() { /* function scope */ }

Examples

const globalVar = "I am global";

function myFunction() {
  const localVar = "I am local";
  console.log(globalVar); // accessible - inner scope sees outer scope
  console.log(localVar);
}

myFunction();
// console.log(localVar); // ReferenceError - not accessible outside the function

if (true) {
  let blockScoped = "only visible in this block";
  var functionScoped = "visible outside the block";
}

// console.log(blockScoped);  // ReferenceError
console.log(functionScoped); // works - var ignores block boundaries

const outer = "outer value";

function level1() {
  const middle = "middle value";
  function level2() {
    const inner = "inner value";
    console.log(outer, middle, inner); // all accessible via the scope chain
  }
  level2();
}

level1();

Best practices

• Keep variables scoped as narrowly as possible - declare them inside the block or function where they are actually used
• Avoid polluting the global scope - wrap code in functions or modules rather than declaring everything globally
• Use let/const for predictable block scoping instead of var's looser function scoping
• Understand the scope chain when debugging "variable is not defined" errors - the variable may simply be out of reach from where you are trying to access it

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation