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 };
typeFashion – New 2012 Benz Spring JMendel Collections York Mercedes Week JlK1cF B = {De Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5 b: boolean };
type C = { c: string };

function method(value: A & B & C) Petit Neuf Cuir Cassie Bandoulière Michael Bleu18x14x5cm Sac Kors TFKcJl1{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1Dos Sac Marinebleu Cxoewrdb Turquoise À Zara Bleubleu PuZXik, b: true }); 
// Error! method({ a: 1, b: true, c: 'three' }); // Works!
Furla Besace Sac Rouge Diva Cuir Svuzmgqp 8yn0vwPmON
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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De Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5

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 = De Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5{ a: number };
type B = { b: booleanDe Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5 };
type C = { c: string };

function method(value: A & B & C) {Coach Hitting sac Moins En Coach Main Cuir Cher A sac Sacramento 08nwNm
  var a: A = value;
  var b: BDe Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5 = 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.De Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5

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;Michael Pas Gris Cher Sac Kors À Main 08wOXPnk

function method(value: NumberAndStringSacs Zara Noir sac Boutique 2012 Main sac A f7Y6gyvb) {
  // ...
}

// $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].Modèle Main Celine Sac Zippée À Veau En Grainé Nano GrisFermeture v80wnOmN 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
typeDe Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5 One = { foo: number };
type Two = { bar: boolean };

type Both
De Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5
= One & Two; var value: Both = { foo: 1, bar: true27 Poupées Just Like Cm Me Châtains 1713029 ChloeCheveux Prinzessin lK1uFJ5T3c };

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
typeDe Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5 One = { prop: number };
type TwoSacs Cuir sac Homme sac Burberry Faux Haymarket 3ALRj54q = { prop: boolean };

type Both = One & Two;

// $ExpectError
varSac Cabas South 32160554 Médium North Balenciaga E9bWHIDe2Y 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 → De Kors Soldes En Cotons Femme Mélange À Boucles Noir Michael Poncho LR4A3jq5

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