Scope

What is Scope?

Scope defines where a variable is accessible in your program.
In JavaScript, there are three main types of scope:

1. Global: accessible everywhere.
2. Function: accessible only inside a function.
3. Block: accessible only inside a block { } (if, for, while…).

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Global Scope

A variable declared outside any function or block is said to be global.
It is accessible from anywhere in the program.

let globalVariable = "I am global";

function showGlobal() {
  console.log(globalVariable);
}

showGlobal(); // I am global

Warning

Too many global variables = hard to maintain code.

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Function Scope

A variable declared inside a function only exists in that function.
We say it has a local scope.

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

test();
// console.log(localVariable); // Error: not defined

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Block Scope

With let and const, variables are limited to the block of code in which they are defined.

if (true) {
  let blockVariable = "Inside the block";
  console.log(blockVariable); // OK
}

// console.log(blockVariable); // Error: not defined

With var, this is not the case → var ignores block scope and behaves like a function-scoped variable.

if (true) {
  var test = "I escape the block";
}
console.log(test); // "I escape the block"

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Scope Summary

Scope type	Keywords involved	Accessible where?
Global	var, let, const	Everywhere in the code (⚠️ avoid too many globals)
Function	var, let, const	Only inside the function
Block	let, const	Only inside the block { } (if, for, while…)

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Best Practices

• Use const by default, and let if the value must change.
• Avoid var (old behavior, source of bugs).
• Minimize global variables → prefer local variables and functions to better organize your code.