Summary - Type inference

Write a variable tracker by combining inference rules.

Summary - Type inference

TypeScript inference rules: let gives a wide type, const gives a literal type, and arrays find the best common type (or union type). Together these rules provide strong type safety with minimal type annotations.

let adet = 0;            // number
const birim = "adet";    // "adet"
let liste = [1, 2, 3];   // number[]

Inference keeps code short while preserving safety.

Exercise Variable tracker program

Define a let named adet with value 0. Define a const named birim with value "adet". Define an array named liste with value [1, 2, 3]. Print adet, birim, and the first element of liste in order with console.log.

Show example solution

let adet = 0;
const birim = "adet";
let liste = [1, 2, 3];
console.log(adet);
console.log(birim);
console.log(liste[0]);

Test yourself

In <code>const x = 100;</code>, what is the inferred type of <code>x</code>?
- any
- 100 (literal type)
- number
- const
Show answer

100 (literal type) — Since <code>const</code> does not change, TypeScript makes the exact value the literal type; the type of <code>x</code> is <code>100</code>, not the general <code>number</code>.

Find the missing part of the union type

let karisik = [1, "iki", true];
// çıkarılan tür: (number | string | ____)[]

Show answer

let karisik = [1, "iki", true];
// çıkarılan tür: (number | string | boolean)[]

Inference summary

let x = 5

Wide type (number)
const x = 5

Narrow literal type (5)
[1, "a"]

Union type array
Type inference

Deriving type from value

Next lesson: Object type

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