Functions are ubiquitous in JavaScript. As expected, Flow propagates types through function calls.

Type Annotating Functions

/* @flow */
function foo(x: string): string { return x; }
var x: number = foo('');

Running Flow produces the following error:

file.js:2:26,31: string
This type is incompatible with
  file.js:3:8,13: number

Open methods

In JavaScript, functions also take an implicit this parameter, and can therefore serve as open methods for objects that have such functions as properties: the this parameter is bound to whatever object the method is called on. Flow understands such behavior and propagates types through this as well. For example, the following code does not typecheck:

/* @flow */
function foo(x) { return this.x; }
var o = { x: 42, f: foo };
var x: string = o.f();
file.js:3:14,15: number
This type is incompatible with
  file.js:4:8,13: string

file.js:4:17,21: call of method f
Too few arguments (expected default/rest parameters in function)
  file.js:2:1,34: function


Functions can take optional and rest parameters, and calls to such functions are checked as expected. For example, the following code typechecks:

/* @flow */
function foo(x, y = false) { }
function bar(z, ...w) { }
bar('h', 'e', 'l', 'l', 'o');

Calls are matched against function signatures following the usual rules of argument matching while taking into account optional/rest parameters.

When checking the body of a function, types of optional parameters are considered optional unless default values are provided.

Too Few Arguments

When you call a function with fewer arguments than it accepts, the void type will be flowed to the missing parameters. If the missing parameter does not accept values of type void then you will get an error.

/* @flow */
function takesANumber(x: number) {}
takesANumber() // Error: undefined passed to x, which expects a number

However, if the missing parameter accepts values of type void then there will be no error.

/* @flow */
function canTakeNoArgs(a: void, b: ?number, c?: number) {}

Too Many Arguments

In JavaScript you can call a function with more arguments than it expects. Flow allows this too. However, there is an easy trick to declare a function can’t take extra arguments.

/* @flow */
function takesOnlyOneNumber(x: number, ...rest: Array<void>) {}
takesOnlyOneNumber(1, 2) // Error: 2 does not have the type void

This is particularly useful when declaring overloads in lib files.

/* @flow */
// The first overload matches 0 args, the second matches 1 arg, the third
// matches 2 args
declare function foo(...rest: Array<void>): string;
declare function foo(a: number, ...rest: Array<void>): string;
declare function foo(a: number, b: number, ...rest: Array<void>): string;

Function-based type annotations

Since functions are first-class values in JavaScript (meaning they can be passed around, like numbers), type annotations may include function types. A function type is of the form (P1: T1, .., Pn: Tn) => U where each Ti is a parameter type, U is the return type, and each Pi is one of the following:

  • an identifier x, suggesting a name for a regular parameter
  • of the form x?, indicating an optional parameter
  • of the form ...x, indicating a rest parameter

There may be at most one rest parameter, which has to appear at the end, and optional parameters must follow regular parameters.

Furthermore, function expressions and function definitions may have parts of their types annotated inline, as seen above. For example, we may have:

function foo (P1: T1, .., Pn: Tn): U { .. }

ES2015 features

Default values assigned to parameters must come after the parameter’s type annotation:

function foo (P1: T1 = V): U { .. }

Arrow functions can be annotated in a similar way:

(P1: T1 .., Pn: Tn): U => { .. }

Polymorphic functions

Functions can be polymorphic, just like polymorphic classes.

/* @flow */
function foo<X>(x: X): X { return x; }

var x: number = foo(0);
var y: string = foo('');

Furthermore, you may have polymorphic methods in polymorphic classes. For example, you may define a List class with a map method:

/* @flow */
class List<T> {
  map<U>(f: (x: T) => U): List<U> { ... }

This means that for every instantiation of T, there is a polymorphic method for objects of type List<T> that, for any instantiation of U, takes a function of type (x: T) => U and returns an object of type List<U>.


Some methods, such as replace() in String and (the polymorphic method) then() in (the polymorphic class) Promise, have multiple signatures to model slightly different use cases that otherwise make sense to group into a single method.

Flow understands such “overloaded” signatures and knows how to apply the correct one for a given call. In fact, the addition operator is a special case of an overloaded function that returns number in some cases and string in others, based on the types of its arguments.

Sometimes, multiple signatures are not needed to express overloading: the signatures can be coalesced using union types. Flow provides the following syntax for union types:

T1 | .. | Tn

is the union of types Ti. Union types are available for general use.

As specific cases, the overloaded signatures of both replace() in String and then() in Promise have been rewritten to use union types, thereby compressing a combinatorial number of signatures into one. In general this is possible whenever return types do not depend on the specific choice and combination of argument types, which is often the case in JavaScript due to lack of overloading support at run time.

Overloading Caution

Overloading is not recommended in general in a dynamic language, because it can be very confusing, and often results in performance penalties. Flow does not yet provide a way to declare overloaded signatures for definitions outside the prelude, so it is not available for general use.

Curiously, there is no actual overloading at run time in JavaScript (since there are no static types at run time). Instead, an overloaded function is implemented by a function that accepts several possible arguments and then does a series of dynamic type tests in its body to dispatch accordingly. This has an interesting effect: since type signatures reflect the truth about implementations, it often turns out that a set of overloaded signatures can be simplified to a single signature using a union type for some parameters.

© 2013–present Facebook Inc.
Licensed under the BSD License.