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Practice

Scope

In JavaScript, scope refers to the range in which variables, functions, and objects can be accessed.

Scope determines where in the code you can access certain variables. JavaScript has several scope rules as outlined below.


Global Scope: A variable with global scope can be accessed from anywhere in the code. These variables are declared at the top level, outside of functions or blocks (e.g., surrounded by {}).

Global Variable
// Declare a global variable
var message = 'Hello, World!';

// Access the global variable
console.log(message); // Output: Hello, World!

function showMessage() {
  message = 'Hello, JavaScript!'; // Modify the global variable
  console.log(message); // Access the global variable
}

// Call the showMessage function
showMessage(); // Output: Hello, JavaScript!

Local Scope: Local scope refers to the scope within a function or a block of code where the variable is declared.

A local variable, which has local scope, is declared inside a function or block and can only be accessed within that function or block, thus preventing conflicts with other parts of the program.

Local and Global Variables
let globalVar = "I'm a global variable!"; // Global variable

function myFunction() {
  let localVar = "I'm a local variable!"; // Local variable
  console.log(globalVar); // Global variable can be accessed within the function
}

console.log(localVar); // Error: localVar is only accessible within myFunction

Block Scope: Variables declared with the let and const keywords have block scope, meaning they can only be accessed within the block (e.g., {}) where they are declared. Variables declared with let can be reassigned, while variables declared with const cannot be reassigned.

Block Scope
if (true) {
  let blockVar = "I'm inside a block!"; // Block scope
  console.log(blockVar); // Can access within the block
}
console.log(blockVar); // Error: blockVar is only accessible within the block

Cautions​

  • Variables declared with var do not have block scope but have function scope.

Example​

Scope Example
let outerVar = "I'm outside!";

function myFunction() {
  let innerVar = "I'm inside!";
  console.log(outerVar); // I'm outside!
  console.log(innerVar); // I'm inside!
}

myFunction();
console.log(outerVar); // I'm outside!
console.log(innerVar); // Error! innerVar is only accessible within myFunction

Scope determines where variables are "visible" and "usable."

Understanding scope rules is crucial to avoid errors in your code.

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