Best common type

Learn the best common type in arrays.

Best common type

When you declare an array, TypeScript finds the common type of the elements. If all elements share a type, that type is used. For example, [1, 2, 3] is inferred as number[].

let sayilar = [1, 2, 3];   // number[]
let metinler = ["a", "b"]; // string[]

The common type is the narrowest type that covers all elements of the array.

Union type in arrays

If array elements have different types, TypeScript infers a union type. For example, ["a", 1] is inferred as (string | number)[]. This means each element can be either a string or a number.

let karisik = ["a", 1];
// type: (string | number)[]

A union type combines different types with the | operator.

Exercise Declare arrays and print first elements

Declare an array named sayilar with value [10, 20, 30]. Declare an array named karisik with value ["metin", 42]. Print the first element of sayilar first, then the first element of karisik, both with console.log.

Show example solution

let sayilar = [10, 20, 30];
let karisik = ["metin", 42];
console.log(sayilar[0]);
console.log(karisik[0]);

Test yourself

In <code>let dizi = [1, 2, 3];</code>, what is the inferred type of <code>dizi</code>?
- number[]
- any[]
- [number, number, number]
- (string | number)[]
Show answer

number[] — Since all elements are numbers, the best common type is <code>number</code>; the array type is inferred as <code>number[]</code>.

Complete the union type

let dizi: (string | ____)[] = ["a", 1];

Show answer

let dizi: (string | number)[] = ["a", 1];

Best common type concepts

[1, 2, 3]

Infers number[]
["a", 1]

Infers (string | number)[]
string | number

Union type
Best common type

Finding the common type in an array

Next lesson: Summary - Type inference

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