Lesson 7

Scope

Scope determines the accessibility (visibility) of variables in different parts of your code.

Variables declared outside any function or block have global scope.

1let globalVar = "I am global";23function test() {4  console.log(globalVar); // Accessible5}67test(); // I am global

Avoid too many global variables to keep code clean and avoid conflicts.

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

1function greet() {2  let msg = "Hello!";3  console.log(msg); // Accessible4}56greet(); // Hello!7console.log(msg);  // Error

Function scoped variables cannot be accessed outside the function.

Variables declared with let or const inside a block { } are accessible only within that block.

1if (true) {2  const pi = 3.14;3  let count = 1;4  console.log(pi); // 3.145}67console.log(pi);    // Error8console.log(count); // Error

Block scope helps prevent accidental access outside the block.

Scope Type	Declared With	Accessibility	Example
Global Scope	`var, let, const`	`Anywhere in the program`	`let a = 10;`
Function Scope	`var, let, const`	`Within the function`	`function foo() {}`
Block Scope	`let, const`	`Within the block {}`	`if (true) { let x; }`

Note: var does not have block scope. It is function-scoped.

Using var instead of let/const
Accessing variables outside their scope
Redeclaring variables in the same scope
Modifying global variables inside functions

Scope controls variable visibility.
let and const provide block scope.
Use the smallest possible scope.
Avoid polluting the global scope.

Experiment with different scopes in the editor.

JavaScript

1let globalMessage = "I am global";
2
3function showMessage() {
4  let localMessage = "I am local";
5  console.log(globalMessage); // Accessible
6  console.log(localMessage);  // Accessible
7}
8
9showMessage();

Output

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