Intersection Types

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.

// @flow
type A = { a: number };
type B = { b: boolean };
type C = { c: string };

function method(value: A & B & C) {
  // ...
}

// $ExpectError
method({ a: 1 }); // Error!
// $ExpectError
method({ a: 1, b: true }); // Error!
method({ a: 1, b: true, c: 'three' }); // Works!

Intersection type syntax

Intersection types are any number of types which are joined by an ampersand &.

Type1 & Type2 & ... & TypeN

You may also add a leading ampersand which is useful when breaking intersection types onto multiple lines.

type Foo =
  & Type1
  & Type2
  & ...
  & TypeN

Each of the members of a intersection type can be any type, even another intersection type.

type Foo = Type1 & Type2;
type Bar = Type3 & Type4;

type Baz = Foo & Bar;

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.

// @flow
type A = { a: number };
type B = { b: boolean };
type C = { c: string };

function method(value: A & B & C) {
  var a: A = value;
  var b: B = 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.

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.

// @flow
type NumberAndString = number & string;

function method(value: NumberAndString) {
  // ...
}

// $ExpectError
method(3.14); // Error!
// $ExpectError
method('hi'); // Error!

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.

// @flow
type One = { foo: number };
type Two = { bar: boolean };

type Both = One & Two;

var value: Both = {
  foo: 1,
  bar: true
};

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.

// @flow
type One = { prop: number };
type Two = { prop: boolean };

type Both = One & Two;

// $ExpectError
var value: Both = {
  prop: 1 // Error!
};