Basic inference

Learn how type inference works.

Basic inference

TypeScript automatically infers a variable's type from the value you assign. This is called type inference. You do not have to write explicit types on every variable; TypeScript determines the type from the assigned value.

let yas = 25;        // infers number
let isim = "Ali";    // infers string
let aktif = true;    // infers boolean

Inference provides type safety with less code.

Inference and type mismatch

The inferred type depends on the type of the assigned value. A string gives string, a number gives number, a boolean gives boolean. The inferred type prevents assigning a different type later.

let x = 5;
x = "metin";   // error: expected number

Inference is based on the first assignment and does not change afterwards.

Exercise Declare variables with inference

Declare three variables without type annotations: assign "merhaba" to mesaj, 10 to adet, and true to aktif. Print all three values in order with console.log.

Show example solution

let mesaj = "merhaba";
let adet = 10;
let aktif = true;
console.log(mesaj);
console.log(adet);
console.log(aktif);

Test yourself

In <code>let isim = "Ali";</code>, what is the inferred type of <code>isim</code>?
- any
- string
- number
- "Ali" (literal)
Show answer

string — When a string is assigned with <code>let</code>, TypeScript infers the wide <code>string</code> type; the literal type <code>"Ali"</code> is only inferred with <code>const</code>.

Write the type matching the inference

let x = "merhaba";
let y: ____ = x;

Show answer

let x = "merhaba";
let y: string = x;

Inference concepts

let x = 5

Infers number
let x = "a"

Infers string
let x = true

Infers boolean
Type inference

Automatic type determination

Next lesson: Inference with const

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