Function Types

Functions have two places where types are applied: Parameters (input) and the return value (output).

// @flow
function concat(a: string, b: string): string {
  return a + b;
}

concat("foo", "bar"); // Works!
// $ExpectError
concat(true, false);  // Error!

Using inference, these types are often optional:

// @flow
function concat(a, b) {
  return a + b;
}

concat("foo", "bar"); // Works!
// $ExpectError
concat(true, false);  // Error!

Sometimes Flow’s inference will create types that are more permissive than you want them to be.

// @flow
function concat(a, b) {
  return a + b;
}

concat("foo", "bar"); // Works!
concat(1, 2);         // Works!

For that reason (and others), it’s useful to write types for important functions.

Syntax of functions

There are three forms of functions that each have their own slightly different syntax.

Function Declarations

Here you can see the syntax for function declarations with and without types added.

function method(str, bool, ...nums) {
  // ...
}

function method(str: string, bool?: boolean, ...nums: Array<number>): void {
  // ...
}

Arrow Functions

Here you can see the syntax for arrow functions with and without types added.

let method = (str, bool, ...nums) => {
  // ...
};

let method = (str: string, bool?: boolean, ...nums: Array<number>): void => {
  // ...
};

Function Types

Here you can see the syntax for writing types that are functions.

(str: string, bool?: boolean, ...nums: Array<number>) => void

You may also optionally leave out the parameter names.

(string, boolean | void, Array<number>) => void

You might use these functions types for something like a callback.

function method(callback: (error: Error | null, value: string | null) => void) {
  // ...
}

Function Parameters

Function parameters can have types by adding a colon : followed by the type after the name of the parameter.

function method(param1: string, param2: boolean) {
  // ...
}

Optional Parameters

You can also have optional parameters by adding a question mark ? after the name of the parameter and before the colon :.

function method(optionalValue?: string) {
  // ...
}

Optional parameters will accept missing, undefined, or matching types. But they will not accept null.

// @flow
function method(optionalValue?: string) {
  // ...
}

method();          // Works.
method(undefined); // Works.
method("string");  // Works.
// $ExpectError
method(null);      // Error!

Rest Parameters

JavaScript also supports having rest parameters or parameters that collect an array of arguments at the end of a list of parameters. These have an ellipsis ... before them.

You can also add type annotations for rest parameters using the same syntax but with an Array.

function method(...args: Array<number>) {
  // ...
}

You can pass as many arguments as you want into a rest parameter.

// @flow
function method(...args: Array<number>) {
  // ...
}

method();        // Works.
method(1);       // Works.
method(1, 2);    // Works.
method(1, 2, 3); // Works.

Note: If you add a type annotation to a rest parameter, it must always explicitly be an Array type.

Function Returns

Function returns can also add a type using a colon : followed by the type after the list of parameters.

function method(): number {
  // ...
}

Return types ensure that every branch of your function returns the same type. This prevents you from accidentally not returning a value under certain conditions.

// @flow
// $ExpectError
function method(): boolean {
  if (Math.random() > 0.5) {
    return true;
  }
}

Async functions implicitly return a promise, so the return type must always be a Promise.

// @flow
async function method(): Promise<number> {
  return 123;
}

Function this

Every function in JavaScript can be called with a special context named this. You can call a function with any context that you want. Flow allows you to annotate the type for this context by adding a special parameter at the start of the function’s parameter list:

// @flow
function method<T>(this: { x: T }) : T {
  return this.x;
}

var num: number = method.call({x : 42});
var str: string = method.call({x : 42}); // error

This parameter has no effect at runtime, and is erased along with types when Flow is transformed into JavaScript. When present, this parameters must always appear at the very beginning of the function’s parameter list, and must have an annotation. Additionally, arrow functions may not have a this parameter annotation, as these functions bind their this parameter at the definition site, rather than the call site.

If an explicit this parameter is not provided, Flow will attempt to infer one based on usage. If this is not mentioned in the body of the function, Flow will infer mixed for its this parameter.

Predicate Functions

Sometimes you will want to move the condition from an if statement into a function:

function concat(a: ?string, b: ?string): string {
  if (a && b) {
    return a + b;
  }
  return '';
}

However, Flow will flag an error in the code below:

function truthy(a, b): boolean {
  return a && b;
}

function concat(a: ?string, b: ?string): string {
  if (truthy(a, b)) {
    // $ExpectError
    return a + b;
  }
  return '';
}

You can fix this by making truthy a predicate function, by using the %checks annotation like so:

function truthy(a, b): boolean %checks {
  return !!a && !!b;
}

function concat(a: ?string, b: ?string): string {
  if (truthy(a, b)) {
    return a + b;
  }
  return '';
}

Limitations of predicate functions

The body of these predicate functions need to be expressions (i.e. local variable declarations are not supported). But it’s possible to call other predicate functions inside a predicate function. For example:

function isString(y): %checks {
  return typeof y === "string";
}

function isNumber(y): %checks {
  return typeof y === "number";
}

function isNumberOrString(y): %checks {
  return isString(y) || isNumber(y);
}

function foo(x): string | number {
  if (isNumberOrString(x)) {
    return x + x;
  } else {
    return x.length; // no error, because Flow infers that x can only be an array
  }
}

foo('a');
foo(5);
foo([]);

Another limitation is on the range of predicates that can be encoded. The refinements that are supported in a predicate function must refer directly to the value that is passed in as an argument to the respective call.

For example, consider the inlined refinement

declare var obj: { n?: number };

if (obj.n) {
  const n: number = obj.n;
}

Here, Flow will let you refine obj.n from ?number to number. Note that the refinement here is on the property n of obj, rather than obj itself.

If you tried to create a predicate function

function bar(a): %checks {
  return a.n;
}

to encode the same condition, then the following refinement would fail

if (bar(obj)) {
  // $ExpectError
  const n: number = obj.n;
}

This is because the only refinements supported through bar would be on obj itself.

Callable Objects

Callable objects can be typed, for example:

type CallableObj = {
  (number, number): number,
  bar: string
};

function add(x, y) {
  return x + y;
}

// $ExpectError
(add: CallableObj);

add.bar = "hello world";

(add: CallableObj);

Function Type

NOTE: For new code prefer any or (...args: Array<any>) => any. Function has become an alias to any and will be deprecated and removed in a future version of Flow.

Sometimes it is useful to write types that accept arbitrary functions, for those you should write () => mixed like this:

function method(func: () => mixed) {
  // ...
}

However, if you need to opt-out of the type checker, and don’t want to go all the way to any, you can instead use (...args: Array<any>) => any. (Note that any is unsafe and should be avoided). For historical reasons, the Function keyword is still available.

For example, the following code will not report any errors:

function method(func: (...args: Array<any>) => any) {
  func(1, 2);     // Works.
  func("1", "2"); // Works.
  func({}, []);   // Works.
}

method(function(a: number, b: number) {
  // ...
});

Neither will this:

function method(obj: Function) {
  obj = 10;
}

method(function(a: number, b: number) {
  // ...
});

You should follow all the same rules as any when using Function.