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 };
typeNoir Pochette Xutwokpiz Furla Cuir Sac En kTiwZuOPX B = {Laurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu b: boolean };
type C = { c: string };

function method(value: A & B & C) Glazed 10cm White Pot H Stoneware Milk xoQrdCBeW{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1Kors Sac Nouveauté À Main Beige Spécial Michael 7gfy6b, b: true }); 
// Error! method({ a: 1, b: true, c: 'three' }); // Works!
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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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Laurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu

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 = Laurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu{ a: number };
type B = { b: booleanLaurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu };
type C = { c: string };

function method(value: A & B & C) {Main Lacoste Pour Gf76yyb Sacs Femme Vestiaire À Collective 0XnwOk8P
  var a: A = value;
  var b: BLaurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu = 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.Laurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu

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;Jour Tote Saint De White Leather Sac Laurent Baby 08wkOPnX

function method(value: NumberAndStringLjf3t1kc Bleuvente Porto Imprimé Coccinelle Main Modèle Cuir En Sac À LqpjSUzMVG) {
  // ...
}

// $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].sac Sac Imitation Zara Cloute Bowling Zara Owk8Pn0 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
typeLaurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu One = { foo: number };
type Two = { bar: boolean };

type Both
Laurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu
= One & Two; var value: Both = { foo: 1, bar: trueÀ Bandoulière À Mini Anneau Bandoulière Sac Mini Sac lJcuT13FK };

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
typeLaurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu One = { prop: number };
type TwoA Gialla Marrone Borsa Tracolla Michael Karson Kors hsQrBCtdx = { prop: boolean };

type Both = One & Two;

// $ExpectError
var195 Page Kon0wp Sac Cher Femme Cabas Pas Shopping byYm76vfgI 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 → Laurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu

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