var and scope
- Node.js course
- Beginner
- Lesson 8/116
- 4 min
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.
Practice 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').
Starter code
// Inside an if (true) block, write let x = 10
// Inside the block, log x
// Outside, log 'done'Sample solution
if (true) {
let x = 10;
console.log(x);
}
console.log("done");Expected output
10 done
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the scope concepts
| Term | Meaning |
|---|---|
var |
Function scope |
let |
Block scope (mutable) |
const |
Block scope (fixed) |
hoisting |
Hoisting |
2 What is the main problem with var?
- It is very slow
- No block scope, has function scope Correct answer
- It cannot be reassigned
- It can only hold numbers
Explanation: var is function-scoped and leaks out of blocks.
3 Block scope with let
if (true) {
____
console.log(x);
}if (true) {
let x = 5;
console.log(x);
}Explanation: var gives function scope, not block scope.