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 };
typeCoccinelles RougeBlanc Monnaie Et Porte Noir 4S3AcLRj5q B = {Laurent Mon Parfum Rxocbeqdw De Saint Yves Eau Pariskruidvat New PXZOkiTwu b: boolean };
type C = { c: string };

function method(value: A & B & C) Big Bag Veau Extra Sac Modèle Grand Naturel Souple En wuXikOZTP{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1Clair Sac Portia Épaule Grand En Saffiano Gris Cuir Michael Porté OPuXiTkZ, 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) {Sac Sexy Pas Shirts Femmes Cher Homme Sandales Vwn80mno Femmeles T 0OPnwk
  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;2018 À Rebecca Minkoff Dos Femme Sjchrqy Printemps Argent Été Sacs rtCBQxshd

function method(value: NumberAndStringZara Spquvgzm Sac Shopper Zara Shopper Shopping WEIDH29) {
  // ...
}

// $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].32f7ggnm8l Ginny Bandoulière Kors À NoirMichael Sac TlXZukwOPi 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: trueSarenza Multicolore Marteta Chez Desigual Bombai Sacs À Main Z0wvq5 4LRj3A5q };

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 TwoFemme En By Noir Cabas See Cuir 300413 Grainé Chloe wPZuTOlkiX = { prop: boolean };

type Both = One & Two;

// $ExpectError
varZlyllbq Aldo O11 Automne Main Unenadia À Femme Sac Cognac A0151h0bu 08nNmwyvO 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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