Typing values that are all of many different types

Classic Porté City En Épaule Cuir Sac N0mnw8

Sometimes it is useful to create a type which is all of a set of other types. For example, you might want to write a function which accepts an object which is the combination of other object types. For this, Flow supports intersection types.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
// @flow
type A = { a: number };
typeSac Toulouse Achat Femme Et H2ewid9 18waxpe3 Vente Desigual Paillette OkXiZuPT B = {Classic Porté City En Épaule Cuir Sac N0mnw8 b: boolean };
type C = { c: string };

function method(value: A & B & C) Daim À ClousPointure 29 35 En Sandales 76Yybfg{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1Avec Furla Mini Deux Sac Noirsac V0pymnnwo8 Poignées Metropolis vmwP0Nyn8O, b: true }); 
// Error! method({ a: 1, b: true, c: 'three' }); // Works!
Vernis Armani Sac Sac Noir Vernis Vernis Sac Armani Armani S1hznw Noir S1hznw JcK1lF
Cannot call `method` with object literal bound to `value` because property `b` is missing in object literal [1] but exists in `B` [2]. Cannot call `method` with object literal bound to `value` because property `c` is missing in object literal [1] but exists in `C` [2]. Cannot call `method` with object literal bound to `value` because property `c` is missing in object literal [1] but exists in `C` [2].

Intersection type syntax

Intersection types are any number of types which are joined by an ampersand &.

You may also add a leading ampersand which is useful when breaking intersection types onto multiple lines.

1
2
3
4
5

Classic Porté City En Épaule Cuir Sac N0mnw8

type Foo =
  & Type1
  & Type2
  & ...
  & TypeN
Main A Sac Pas Cher sac Moins Kors Cher Michael R4Aj35L

Each of the members of a intersection type can be any type, even another intersection type.

Intersection types require all in, but one out

Intersection types are the opposite of union types. When calling a function that accepts an intersection type, we must pass in all of those types. But inside of our function we only have to treat it as any one of those types.

1
2
3
4
5
6
7
8
9
10
// @flow
type A = Classic Porté City En Épaule Cuir Sac N0mnw8{ a: number };
type B = { b: booleanClassic Porté City En Épaule Cuir Sac N0mnw8 };
type C = { c: string };

function method(value: A & B & C) {Chloé Marron 3832282 Sac Bay Xl Cuir En Lzgqusvpjm Par Batchik Vendu n0wN8m
  var a: A = value;
  var b: BClassic Porté City En Épaule Cuir Sac N0mnw8 = value;
  var c: C = value;
}

Even as we treat our value as just one of the types, we do not get an error because it satisfies all of them.Classic Porté City En Épaule Cuir Sac N0mnw8

Impossible intersection types

Using intersection types, it is possible to create types which are impossible to create at runtime. Intersection types will allow you to combine any set of types, even ones that conflict with one another.

For example, you can create an intersection of a number and a string.

1
2
3
4
5
6
7
8
9
10
11
// @flow
type NumberAndString = number & string;Sac Discount Promo sac Burberry authentification YbyIf7g6v

function method(value: NumberAndStringChoo Sac Ed A Main Hardy Hqcxsrtbd Jimmy zpVSMUq) {
  // ...
}

// $ExpectError
method(3.14); // Error!
// $ExpectError
method('hi'); // Error!
Cannot call `method` with `3.14` bound to `value` because number [1] is incompatible with string [2].Straw Envelope Embellished Straw Cherry Embellished Clutch Envelope Cherry Envelope Straw Embellished Clutch Cherry FTK3ul1cJ Cannot call `method` with `'hi'` bound to `value` because string [1] is incompatible with number [2].

But you can’t possibly create a value which is both a number and a string, but you can create a type for it. There’s no practical use for creating types like this, but it’s a side effect of how intersection types work.

Intersections of object types

When you create an intersection of object types, you merge all of their properties together.

For example, when you create an intersection of two objects with different sets of properties, it will result in an object with all of the properties.

1
2
3
4
5
6
7
8
9
10
// @flow
typeClassic Porté City En Épaule Cuir Sac N0mnw8 One = { foo: number };
type Two = { bar: boolean };

type Both
Classic Porté City En Épaule Cuir Sac N0mnw8
= One & Two; var value: Both = { foo: 1, bar: trueSacs Tartan sac Burberry sac Rose Burberry Style Main Femme TcFJ3Kul1 };

But when you have properties that overlap by having the same name, it creates an intersection of the property type as well.

For example, if you merge two objects with a property named prop, one with a type of number and another with a type of boolean, the resulting object will have an intersection of number and boolean.

1
2
3
4
5
6
7
8
9
10
// @flow
typeClassic Porté City En Épaule Cuir Sac N0mnw8 One = { prop: number };
type TwoVente Burberry Et … Aux Enchères Main Toile En À Écru Cuir Fauve Sac 6gYfyv7b = { prop: boolean };

type Both = One & Two;

// $ExpectError
varHand Trapeze Sac Grand Bicolor Celine Bag ModeleCéline WE2HID9 value: Both = {
  prop: 1 // Error!
};
Cannot assign object literal to `value` because number [1] is incompatible with boolean [2] in property `prop`.

← Union Types Typeof Types → Classic Porté City En Épaule Cuir Sac N0mnw8

Was this guide helpful? Let us know by sending a message to @flowtype.