JavaScript Scope

Scope determines the accessibility of variables.

There are 3 types of scope in JavaScript variables. They are:

Block scope
Function scope
Global scope

Block Scope

Variables declared with let and const have block scope. Variables declared inside a { } block cannot be accessed from outside of it.

Example:

{ let x = 10; const y = 20; console.log(x, y); // Accessible here } // console.log(x, y); // Error: x and y are not defined

Local Scope

Variables declared inside a function are local to that function.

Example:

function myFunction() { let localVar = ‘I am local’; console.log(localVar); } myFunction(); // console.log(localVar); // Error

Function Scope

Variables declared with var have function scope.

Example:

function testFunctionScope() { var testVar = ‘Hello’; if (true) { var testVar = ‘Hi’; // Same variable console.log(testVar); // Hi } console.log(testVar); // Hi } testFunctionScope();

Global JavaScript Variables

Global variables are accessible throughout the program.

Example:

var globalVar = ‘I am global’; function accessGlobal() { console.log(globalVar); } accessGlobal();

Global Scope

Global scope refers to variables that are declared outside of any function.When defined outside of a block, variables declared using var, let, and const are fairly identical.

Example:

var x = 3; //Global scope let x = 3; //Global scope const x = 3; //Global scope

Automatically Global

If a variable is assigned without declaring it, it becomes global.

Example:

function makeGlobal() { globalAuto = ‘Global by mistake’; } makeGlobal(); console.log(globalAuto); // Accessible globally

Strict Mode

Strict mode helps avoid common mistakes by enforcing stricter parsing and error handling.

Example:

‘use strict’; function strictFunction() { undeclaredVar = ‘This will throw an error’; // ReferenceError } strictFunction();

Global Variables in HTML

Global variables can be accessed as properties of the window object in browsers.

Example:

var globalVar = ‘Hello’; console.log(window.globalVar); // Hello

The Lifetime of JavaScript Variables

Variables have a lifetime based on their scope:

Global variables remain active as long as the application is in use.

Local variables are only present whilst the function is running.

Function Arguments

Function arguments behave like local variables within the function.

Example:

function addNumbers(a, b) { return a + b; } console.log(addNumbers(5, 10)); // 15

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JavaScript Scope