Variable annotation

Learn to give explicit types to variables.

Variable annotation

In TypeScript you give a variable an explicit type by writing : type after its name. This is an explicit type annotation. Without it, TypeScript infers the type from the assigned value.

let yas: number = 25;   // explicit
let isim = "Ali";       // inferred: string

Explicit types improve code readability and safety.

When is an explicit type needed?

Since TypeScript can infer types automatically, an explicit type is not needed on every variable. However, when declaring a variable without an initial value, an explicit type is required. It can also be preferred for code clarity.

let x: number;    // type required
x = 10;

Without an initial value TypeScript cannot know the type, so you must provide an explicit type.

Exercise Declare variables with explicit types

Declare a variable named yas of type number with value 25. Declare a variable named isim of type string with value "Ali". Print yas first, then isim, both with console.log.

Show example solution

let yas: number = 25;
let isim: string = "Ali";
console.log(yas);
console.log(isim);

Test yourself

What does <code>let x: string = "merhaba";</code> do?
- Converts x to string at runtime
- Prevents reassignment of x
- Explicitly declares x as string type
- Makes x a constant
Show answer

Explicitly declares x as string type — Adding <code>: string</code> explicitly declares the variable's type; it does not change runtime behavior, only strengthens compile-time checking.

Add the variable type

let sayi: ____ = 42;

Show answer

let sayi: number = 42;

Variable annotation concepts

let x: string

Explicit type declaration
let x = "a"

Type inference
let x: number = 5

Explicit type and value
Değişken annotation

Giving a type to a variable

Next lesson: Parameter annotation

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