Master JavaScript Scopes in 6 Minutes

Understanding the different scopes in JavaScript is essential for writing clean, efficient, and bug-free code. Let's explore the six types of scope in JavaScript: Global Scope, Local Scope (Function Scope), Block Scope, Lexical Scope, Module Scope, and Script Scope.

Global Scope

Variables declared outside any function or block are in the global scope. These variables are accessible from anywhere in the code.

var globalVar = 'I am a global variable'

function showGlobalVar() {
  console.log(globalVar) // I am a global variable
}

Local Scope (Function Scope)

Variables declared inside a function are in the local scope. These variables are only accessible within the function.

function showLocalVar() {
  var localVar = 'I am a local variable'
  console.log(localVar) // I am a local variable
}

console.log(localVar) // ReferenceError: localVar is not defined

Block Scope

Variables declared with let or const within a block (denoted by : e.g., if statement, for loop, or even itself) are block-scoped. They are only accessible within that block.

{
  let blockVar = 'I am a block variable'
  console.log(blockVar) // I am a block variable
}

console.log(blockVar) // ReferenceError: blockVar is not defined

Lexical Scope

Lexical scope refers to the visibility of variables in nested functions. Inner functions have access to variables declared in their outer function.

function outerFunction() {
  var outerVar = 'I am an outer variable'

  function innerFunction() {
    console.log(outerVar) // I am an outer variable
  }

  innerFunction()
}

Module Scope

In modern JavaScript (ES6 and beyond), modules are files that have their own scope. Variables, functions, and classes declared in a module are not accessible outside the module unless explicitly exported.

// module.js
var moduleVar = 'I am a module variable'

export function showModuleVar() {
  console.log(moduleVar) // I am a module variable
}

// index.js
import { showModuleVar } from './module'

showModuleVar() // I am a module variable

Script Scope

In browsers, JavaScript running in the context of a <script> tag without the type="module" attribute has script scope. Variables declared in script scope are similar to global scope, but they are confined to the script.

<script>
  var scriptScopedVar = 'I am a script-scoped variable'

  function showScriptScopedVar() {
    console.log(scriptScopedVar) // I am a script-scoped variable
  }
</script>

<script>
  console.log(scriptScopedVar) // I am a script-scoped variable
</script>

Understanding these scopes will help you manage variable accessibility and lifetime more effectively, leading to better-structured and more maintainable code.

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