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Flow has types for all of the JavaScript primitive types.

See Built-in Types for more information and examples.


The any type is a supertype and subtype of all types.

Code that uses any is effectively unchecked, and should be avoided when another type can be used instead.

It can be useful to opt out of type checking, however. In particular, it is very useful when converting an existing code base to use types.

It is also occasionally necessary to bypass the type checker. There are a (decreasing) number of JS idioms which Flow is not able to type statically. In these instances, it is practical and reasonable to use any.

See Built-in Types for more about any.


The mixed type is a supertype of all types.

This type is particularly useful when paired with dynamic type tests. For example, you can use mixed to annotate a value of an unknown type, and Flow will ensure that you perform the necessary type tests to use the value safely.

See Built-in Types for more about mixed.


This type describes JavaScript array objects and the type of the elements contained within the array.

Indexing into an array with type Array<T> will always yield a value with type T. That is, Flow assumes arrays are dense and does not do bounds checking.

let array: number[] = [1, 2, 3.14, 42];
let theAnswer: number = array[3]; // 42
let offTheEnd: number = array[100]; // No error

let array2: Array<string> = ["an alternate", "syntax", "for arrays"];


Tuple types are a kind of array type particularly suited to describe finite, heterogeneous collections.

let tuple: [string, number, boolean] = ["foo", 0, true];

// Indexing into the array will return the type at a given index.
(tuple[0]: string);
(tuple[1]: string);

// Indexing into an statically unknown index will return a general type.
declare var unknownNumber: number;
// `void` is none of `string`, `number`, or `boolean`
(tuple[unknownNumber]: void);
(tuple[unknownNumber]: string|number|boolean); // OK

// Values written must be compatible with the type at that index.
tuple[1] = -1;
tuple[0] = false;

See Arrays for more information and examples.


This type describes any object values that match a specified shape.

let object: {foo: string, bar: number} = {foo: "foo", bar: 0};
(object.foo: string);

// Property writes must be compatible with the declared type.
object.bar = "bar";

Objects as maps

Objects are often used as lookup tables, or maps. While the Map type is more suitable for this use case, Flow does support this common idiom.

let coolRating: {[id:string]: number} = {};
coolRating["sam"] = 10; // Yes, it's a 0-10 scale.

Callable objects

Functions are also objects, and may have other props. Flow models this as an object type with a callable property.

function makeCallable(): { (x: number): string; foo: number } {
  function callable(number) {
    return number.toFixed(2);
  callable.foo = 123;
  return callable;

var callable = makeCallable();

var callableReturn: string = callable(Math.PI); // "3.14"
var callableFoo: number = callable.foo; // 123

The Object type

The Object type is a supertype of all object types. A value of this type supports property access by any property name, and will return a value with type any.

Like any, this type should be used sparingly.

var anyObject: Object = {};
anyObject.foo.bar.baz; // OK

See Objects for more information and examples.


Functions take zero or more arguments and optionally return a value. In addition to "standard" function declarations, Flow supports arrow functions, async functions, and generator functions.

function greatestCommonDivisor(a: number, b: number): number {
  if (!b) {
    return a;

  return greatestCommonDivisor(b, a % b);

// Annotations included for example purposes only.
[1, 2, 3].map((num: number): number => num * 2)

async function getFriendNames(
  friendIDs: Promise<number[]>,
  getFriendName: (id: number) => Promise<string>,
): Promise<string[]> {
  var ids = await friendIDs;
  var names = await Promise.all(ids.map(getFriendName));
  return names;

function *infinity(): Generator<number,void,void> {
  var n = 0;
  while (true) {
    yield n++;

The Function type

The Function type is a supertype of all function types. A value of this type may be called with any number of any type of parameters, and will return a value with type any.

Like any, this type should be used sparingly.

var anyFunction: Function = () => {};
anyFunction("foo", "bar").baz.quux; // OK

See Functions for more information and examples.


Defining a class also defines a type, which can be used to annotate instances of that class.

class MyClass {
  foo: string;
  constructor(foo: string) {
    this.foo = foo;
  bar(): string {
    return this.foo;

var myInstance: MyClass = new MyClass("foo");
(myInstance.foo: string);
(myInstance.bar(): string);


Classes are nominally typed in Flow. That means that two classes are only compatible if they have an explicit subtyping relationship, via extends. It is often useful to describe a set of types which are structurally similar.

interface Fooable {
  foo(): string;

class AFoo {
  foo() { return "foo from A" };

class BFoo {
  foo() { return "foo from B" };

(new AFoo: Fooable);
(new BFoo: Fooable);

Sugar-free classes

ES2015 classes formalize the common practice of simulating class-like inheritance with functions and prototypes. Flow does have limited support for this pattern as well, but the ES2015 class syntax is highly recommended.

function DietClass(foo: string) {
  this.foo = foo;

DietClass.prototype.bar = function() {
  return this.foo;

var myDietInstance: DietClass = new DietClass("foo");
(myDietInstance.foo: string);
(myDietInstance.bar(): string);

The Class<T> type

Remember that the name of the class as a type annotation represents instances of that class. Given a type T representing instances of a class C, the type Class<T> is the type of the class C.

var myClass: Class<MyClass> = MyClass;
var myInstance2 = new myClass("foo");

See Classes for more information and examples.

Type aliases

Instead of writing out a potentially complex type multiple times, define a type alias instead.

type ObjectWithManyProperties = {
  foo: string,
  bar: number,
  baz: boolean,
  qux: (foo: string, bar: number) => boolean;

See Type Aliases for more information and examples.


Generics make it possible to abstract over types. A common example is container classes, which store, traverse, and retrieve data without knowledge of specifically what is inside.

Generic object types can be specified using a type alias.

type GenericObject<T> = { foo: T };
var numberObject: GenericObject<number> = { foo: 0 };
var stringObject: GenericObject<string> = { foo: "foo" };

Type parameters for classes are specified in the class declaration.

class GenericClass<T> {
  x: T;
  constructor(x: T) {
    this.x = x;

var numberInstance: GenericClass<number> = new GenericClass(0);
var stringInstance: GenericClass<string> = new GenericClass("");

Type parameters for functions are specified in the function declaration.

function findMax<T>(arr: T[], compare: (a: T, b: T) => number) {
  var sorted = arr.sort(compare);
  return sorted[sorted.length - 1];