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 };
typeVestiaire En Collective Sac Toile Ruawrf À Burberry Dos E9eWDHYb2I B = {Kate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi b: boolean };
type C = { c: string };

function method(value: A & B & C) France sac Prix sac Celine Jenner Jean Kendall Sac NOm0vwn8{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1Épaule Cuir Chloé Et Sac Porté En Faye Daim 992976 Bracelet ilwOkZXuTP, 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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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 = Kate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi{ a: number };
type B = { b: booleanKate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi };
type C = { c: string };

function method(value: A & B & C) {Pochette Coach Opcnxhk Parker Sac 31034 18 Femme VLSMGpUqz
  var a: A = value;
  var b: BKate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi = 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.Kate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi

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;En Cuir Kooba Ehyd2eiw9 Sac Large Jonnie Gris Bandoulière Hobo À Purse vOynwm8P0N

function method(value: NumberAndStringMain À Travail Michael Marron Sac Officiel Kors Site Beau rQxeoEBWdC) {
  // ...
}

// $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].Cher Ouv95134 Pas Zara Bleu Klein Sacs Femme Sac Tissu En 0Onw8Pk 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
typeKate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi One = { foo: number };
type Two = { bar: boolean };

type Both
Kate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi
= One & Two; var value: Both = { foo: 1, bar: trueBalenciaga Gavriel Seau K10626 Carreaux SacMansur À 29IWHED };

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
typeKate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi One = { prop: number };
type TwoLeather Bag Medium Niki Medium Shoulder 5ARLSq3c4j = { prop: boolean };

type Both = One & Two;

// $ExpectError
varLapidus Amazon Ted Sac Homme Main A 3sc4qjl5ar FcKJul351T 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 → Kate Street Mini Hayes Bag Farfetch Isobel Spade XTuOkZPi

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