var and scope

Learn var's function scope and hoisting.

var and scope

var is JavaScript's old variable declaration keyword. Variables declared with var have function scope; even if they are declared inside a { } block, they are accessible from outside the block.

This can cause unexpected bugs. For example, if you declare var x = 5 inside an if block, x is also visible outside the block.

That is why modern Node.js code prefers let and const.

Function and block scope

Function scope: a variable is visible throughout the function where it is declared. Block scope keeps a variable only inside the { } block where it is declared.

var is function-scoped: a var x declared inside an if block is also visible outside. let and const are block-scoped: they cannot be accessed outside the block.

This is why modern Node.js code prefers let and const; unexpected leaks are prevented.

Exercise Show var vs let

Inside an if (true) block, declare let x = 10. Inside the block, call console.log(x). Outside the block, call console.log('done').

Show example solution

if (true) {
  let x = 10;
  console.log(x);
}
console.log("done");

Test yourself

Match the scope concepts

var

Function scope
let

Block scope (mutable)
const

Block scope (fixed)
hoisting

Hoisting
What is the main problem with var?
- It is very slow
- No block scope, has function scope
- It cannot be reassigned
- It can only hold numbers
Show answer

No block scope, has function scope — var is function-scoped and leaks out of blocks.

Block scope with let

if (true) {
  ____
  console.log(x);
}

Show answer

if (true) {
  let x = 5;
  console.log(x);
}

Next lesson: Type inference

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