Composition with alias

Learn to create new types by combining aliases.

Composition with aliases

You can combine types with | (union) to create a new alias. This is useful when a value can be one of several types. Aliases can also reference other aliases.

type ID = number | string;
type Durum = "acik" | "kapali";

A union type allows a value to be one of several types.

Deriving aliases from aliases

An alias can use another alias in its definition. This lets you build complex types from simpler ones, making code modular.

type ID = number;
type Etiket = string;
type Kayit = ID | Etiket;

You can create hierarchical type definitions by combining aliases.

Exercise Compose aliases

Define a type alias named ID (number). Define a type alias named Etiket (string). Define a type alias named Kayit (ID | Etiket union). Declare a variable named kayit of type Kayit with value 42. Print the value of kayit with console.log.

Show example solution

type ID = number;
type Etiket = string;
type Kayit = ID | Etiket;
let kayit: Kayit = 42;
console.log(kayit);

Test yourself

What kind of type does <code>type Sonuc = number | string</code> create?
- A type that is only number
- A type that can be either number or string
- A type that is only string
- A type that is both number and string simultaneously
Show answer

A type that can be either number or string — The <code>|</code> operator creates a union type; the value can be either number or string, not both at the same time.

Add the second part of the union type

type Durum = "acik" | ____;

Show answer

type Durum = "acik" | "kapali";

Composition concepts

type X = A | B

Union type
type X = A (A bir alias)

Deriving alias from alias
|

Type union operator
Composition

Combining aliases

Next lesson: Alias vs interface

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