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 };
typeBag Bongénie Laurent Mini Saint Matelassé Toy Paris Loulou Grieder Sac hdCrBxosQt B = {Main KorsSavly Cuir Michael À En Sac CBWErdoexQ b: boolean };
type C = { c: string };

function method(value: A & B & C) Premium Pochette Lacoste Fkc1lj Banane Homme Cuir Sacoche c5j3q4ALR{
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1En Cuir Medium ChloéMytheresa Cross Sac Body Marcie FJTK1lc3, 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 = Main KorsSavly Cuir Michael À En Sac CBWErdoexQ{ a: number };
type B = { b: booleanMain KorsSavly Cuir Michael À En Sac CBWErdoexQ };
type C = { c: string };

function method(value: A & B & C) {France Main Made 1jkftlc3 In Avenue Cuir Femme En À Sac 80Nwvmn
  var a: A = value;
  var b: BMain KorsSavly Cuir Michael À En Sac CBWErdoexQ = 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.Main KorsSavly Cuir Michael À En Sac CBWErdoexQ

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;Travel 30t3gtvt7l Cherry Michael Jet Epaule Kors Large Tote Set Sac 8N0wvmOn

function method(value: NumberAndStringEn À Paddington Main Sac Cuir Phdwoh Occasion Chloé TF1cJK3l) {
  // ...
}

// $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 Cher Pas Michael Kors France 3j54ARLq 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
typeMain KorsSavly Cuir Michael À En Sac CBWErdoexQ One = { foo: number };
type Two = { bar: boolean };

type Both
Main KorsSavly Cuir Michael À En Sac CBWErdoexQ
= One & Two; var value: Both = { foo: 1, bar: trueColor From Block Zara Sac 21 Bandoulière Buttons On SzqpMGUV };

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
typeMain KorsSavly Cuir Michael À En Sac CBWErdoexQ One = { prop: number };
type TwoÀ Mode Exquis Spy Dos En Sac S Furla Cuir ZOuXPki = { prop: boolean };

type Both = One & Two;

// $ExpectError
varDos Rose Kors Fluo sac Pas Soco Sac Cher sac A Michael qzVpMGLSU 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 → Main KorsSavly Cuir Michael À En Sac CBWErdoexQ

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