JS Scope

JS Scope

Scope determines where variables are accessible in your code. JavaScript has global scope, function scope, and block scope.

Global Scope

Variables declared outside functions or blocks are globally accessible.

let site = "W4";

function show() {
  console.log(site);
}

show();

Function Scope

Variables declared inside a function are only accessible within that function.

function test() {
  let message = "Inside function";
  console.log(message);
}

test();
// console.log(message); // Error

Block Scope

Variables declared with let and const are limited to the block in which they are defined.

if (true) {
  let value = 10;
  const pi = 3.14;
}

// console.log(value); // Error

JS Code Blocks

A block is any code inside curly braces { }. Blocks define scope for let and const.

{
  let x = 5;
}
// console.log(x); // Error

However, var does NOT respect block scope.

{
  var y = 20;
}

console.log(y); // 20

JS Hoisting

Hoisting is JavaScript's behavior of moving declarations to the top of their scope before execution.

Function Hoisting

greet();

function greet() {
  console.log("Hello");
}

Variable Hoisting

var declarations are hoisted but initialized as undefined.

console.log(a); // undefined
var a = 5;

let and const are also hoisted but remain in the Temporal Dead Zone (TDZ) until declared.

// console.log(b); // Error
let b = 10;

JS Strict Mode

Strict mode enables a stricter parsing and error handling in your JavaScript code.

"use strict";

x = 10; // Error: variable not declared

Strict mode prevents accidental global variables and disallows certain unsafe features.

Strict Mode in Functions

function test() {
  "use strict";
  let value = 5;
  console.log(value);
}

Best Practice

Use let and const instead of var. Enable strict mode in modern projects or rely on ES modules, which use strict mode by default.

Next Step

Continue with JS Dates to learn how JavaScript works with time and date values.