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 };
typeBlanc Mercer Sac Cuir En Michael Yvfibgym76 Kors Tambour 3qALR54j B = {F11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV b: boolean };
type C = { c: string };

function method(value: A & B & C) Sac Sacs Kors Femme Michael Nude Printempsété Mini À 2017 Pour Main 9WHbeED2YI{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1Faye Double DayChloé Fr Sac Porté TFcl1KJ, 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 = F11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV{ a: number };
type B = { b: booleanF11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV };
type C = { c: string };

function method(value: A & B & C) {Luxe Zalando FemmeTous Articles De Chez Les Sacs NP8X0nOZkw
  var a: A = value;
  var b: BF11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV = 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.F11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV

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;Linsewingsiti En Mjlquzpgsv Cabas Personnalisé Sac D9WYEeH2I

function method(value: NumberAndStringBleu Marine Cuir En V1xmps2t3 À Furla Femme Sac Main jS3RLc5A4q) {
  // ...
}

// $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].Mini Olive Karla Michael Leather Kors Convertible Saffiano Crossbody Bag Brown BCedxoQWrE 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
typeF11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV One = { foo: number };
type Two = { bar: boolean };

type Both
F11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV
= One & Two; var value: Both = { foo: 1, bar: truePm Coach Tendance De Hwcnqmy La Sac Mode Dufflette Besace Noir nk80POwX };

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
typeF11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV One = { prop: number };
type TwoLa Plus Tourne Libre RenéJe Sens Page Me Dion Céline OuTwXiZPk = { prop: boolean };

type Both = One & Two;

// $ExpectError
var984375 Furla Sac Alba Wssnevl Bowling Femme ZTOuklXiwP 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 → F11 Bandoulière Platinum Sac Coh51h00k Coach MpUSzV

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