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Type Aliases

When you have complicated types that you want to reuse in multiple places, you can alias them in Flow using a type alias.

1type MyObject = {2  foo: number,3  bar: boolean,4  baz: string,5};

These type aliases can be used anywhere a type can be used.

1type MyObject = {2  // ...3};4
5const val: MyObject = { /* ... */ };6function method(val: MyObject) { /* ... */ }7class Foo { constructor(val: MyObject) { /* ... */ } }

Type aliases are just that: aliases. They don't create a different type, just another name for one. This means that a type alias is completely interchangeable with the type it is equal to.

1type MyNumber = number;2declare const x: MyNumber;3declare function foo(x: number): void;4foo(x); // ok, because MyNumber = number

Opaque type aliases offer an alternative for when you don't want to treat the types as the same.

Type Alias Syntax​

Type aliases are created using the keyword type followed by its name, an equals sign =, and a type definition.

type Alias = Type;

Any type can appear inside a type alias.

1type NumberAlias = number;2type ObjectAlias = {3  property: string,4  method(): number,5};6type UnionAlias = 1 | 2 | 3;7type AliasAlias = ObjectAlias;

Type Alias Generics​

Type aliases can also have their own generics.

1type MyObject<A, B, C> = {2  property: A,3  method(val: B): C,4};

Type alias generics are parameterized. When you use a type alias you need to pass parameters for each of its generics.

1type MyObject<A, B, C> = {2  foo: A,3  bar: B,4  baz: C,5};6
7var val: MyObject<number, boolean, string> = {8  foo: 1,9  bar: true,10  baz: 'three',11};