JavaScript Scope

Scope decides where a variable can be used. A variable made in one place may not be visible in another place.

---

Global and local scope

let globalVar = "global";

function test() {
  let localVar = "local";
  console.log(globalVar);
  console.log(localVar);
}

test();
console.log(localVar);

A variable declared outside every function is global, and can be used everywhere. A variable declared inside a function is local, and exists only in that function.

---

Block scope

A block is the code inside curly braces { }, for example in an if or a loop. let and const stay inside the block. var ignores blocks.

if (true) {
  let a = 1;
  var b = 2;
}

console.log(b);
console.log(a);

---

Hoisting

JavaScript moves declarations to the top of their scope before running the code. For var the variable exists, but its value is undefined until the line is reached.

console.log(x);
var x = 5;
console.log(x);

let and const are also hoisted, but you cannot use them before the line where they are declared. This gap is called the temporal dead zone.

console.log(y);
let y = 5;

Function declarations are hoisted completely, so you can call them before they appear in the code.

---

Nested scope

An inner function can use the variables of its outer function, but not the other way round. This is what makes closures work.

function outer() {
  let message = "Hi";
  function inner() {
    console.log(message);   // can see message
  }
  inner();
}
outer();

---

Summary

• Global variables are visible everywhere, local variables only in their function.
• let and const are block scoped, var is function scoped.
• Declare variables before you use them.
• Next: JavaScript Data Types.

• Previous Page
• Next Page

---

Exercises & Assignments
No Content Found.

Interview Questions & Answers
No Content Found.