let and const

Learn the difference between let and const and type inference.

The difference between let and const

A variable declared with let can be reassigned later; one declared with const cannot be changed after its initial value. Which you use depends on whether the variable will change.

let yas = 25;
yas = 26;  // valid

const PI = 3.14;
PI = 3;  // error

Prefer const when possible; it prevents accidental changes.

Type inference

When you give a variable an initial value, TypeScript infers the type from that value automatically even without a type annotation. This is called type inference.

let sehir = "İzmir";   // string
const PI = 3.14;       // number
let aktif = true;      // boolean

Type inference keeps code short, but writing the type explicitly is also fine.

Exercise const and let together

Declare a const variable PI with value 3.14 and type number. Declare a let variable yaricap with value 5 and type number. Reassign yaricap to 10. Then print PI * yaricap * yaricap with console.log.

Show example solution

const PI: number = 3.14;
let yaricap: number = 5;
yaricap = 10;
console.log(PI * yaricap * yaricap);

Test yourself

Which line of the following code raises an error?
```
const PI = 3.14;
let x = 5;
PI = 3;
x = 10;
```
- Line 2
- Line 3
- Line 4
- Line 1
Show answer

Line 3 — A variable declared with const cannot be reassigned, while one declared with let can. Therefore line 3 (PI = 3;) raises an error.

Choose the right keyword

____ fiyat = 100;
fiyat = 120;

Show answer

let fiyat = 100;
fiyat = 120;

let and const concepts

let

reassignable
const

cannot be reassigned
let x = 5

type inference: number
const y = "a"

type inference: string

Next lesson: Summary - Primitive types

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