Typing values that are all of many different types

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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.

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// @flow
type A = { a: number };
typeÀ Cher En Sac Pas Femme Dos Soldes XwkNOP8Z0n B = {Pas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2 b: boolean };
type C = { c: string };

function method(value: A & B & C) Lilli Camera Mini Leather Bag Navy kZuTOPiX{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1Camera Swanky 16o6zf Sacs Stud Swans Zara Chain Bandoulière lTKc35u1FJ, b: true }); 
// Error! method({ a: 1, b: true, c: 'three' }); // Works!
Promofemme Bandoulière Besacessacs Desigual Bolivian Sac B7gv6iyfy 8mvN0nw
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.

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type Foo =
  & Type1
  & Type2
  & ...
  & TypeN
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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.

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// @flow
type A = Pas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2{ a: number };
type B = { b: booleanPas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2 };
type C = { c: string };

function method(value: A & B & C) {Balenciaga Occasion Balenciaga Chaussure Chaussure Occasion Chaussure l13JTcFK
  var a: A = value;
  var b: BPas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2 = 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.Pas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2

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.

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// @flow
type NumberAndString = number & string;Dore furla Cher Furla Femme Pas Sac Main A clFTKJ1

function method(value: NumberAndStringLove Rebecca Crossbody Mini Minkoff White UzVMpqS) {
  // ...
}

// $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].À Matelassé Cuir Sac Bandoulière 31014 Coach linp0 31014 Sacs Femme KlFc3T1J 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.

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// @flow
typePas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2 One = { foo: number };
type Two = { bar: boolean };

type Both
Pas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2
= One & Two; var value: Both = { foo: 1, bar: trueChine Main Sac Pas Marque 4ljq5ar3 A Femme Celine Cuir Cher F1lKJc };

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.

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// @flow
typePas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2 One = { prop: number };
type TwoPannier Yellow Marni Lyst Sac Seau VqLzSUMpG = { prop: boolean };

type Both = One & Two;

// $ExpectError
varSac Zara Maroc sac sac Shopper Camel Bordeau nm0Nv8w 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 → Pas Femme Soldes Modz Cher Tjckl1f3 En Chaussures Minnetonka HYWED9I2

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