Summary - Type inference
- TypeScript course
- Beginner
- Lesson 37/89
- 4 min
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.
Practice 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.
Starter code
// adet adında bir let tanımla, değeri 0
// birim adında bir const tanımla, değeri "adet"
// liste adında bir dizi tanımla, üç sayı içersin
// adet, birim ve liste'nin ilk öğesini sırayla yazdırSample solution
let adet = 0;
const birim = "adet";
let liste = [1, 2, 3];
console.log(adet);
console.log(birim);
console.log(liste[0]);Expected output
0 adet 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In <code>const x = 100;</code>, what is the inferred type of <code>x</code>?
- any
- 100 (literal type) Correct answer
- number
- const
Explanation: 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>.
2 Find the missing part of the union type
let karisik = [1, "iki", true];
// çıkarılan tür: (number | string | ____)[]let karisik = [1, "iki", true];
// çıkarılan tür: (number | string | boolean)[]Explanation: The array contains a number, a string, and a boolean, so the union type is <code>(number | string | boolean)[]</code>; the third type is <code>boolean</code>.
3 Inference summary
| Term | Meaning |
|---|---|
let x = 5 |
Wide type (number) |
const x = 5 |
Narrow literal type (5) |
[1, "a"] |
Union type array |
Type inference |
Deriving type from value |