Dive into Flow

Run check

To enable flow checking:

; flow check

To check files without adding flow header, specify them in [include] in .flowconfig.

Alternatively, to check all files including files without flow header /* @flow */, set all = true in [option] in .flowconfig or run flow via flow check --all.

Currently WebStorm 2016.3.1 only recognize flow header, not settings in .flowconfig.

For large projects, run flow as a sever to check incrementally:

; flow
; flow # stop the server

Weak mode

To introduce type checking for existing code gradually:

/* @flow weak */

In weak mode, Flow will do much less type inference, skipping checks for unannotated variables not within functions.


flow-typed provides libdef for specifying types for libraries separately.

; npm install
; flow-typed install

Example of libdef:

// flow-typed/lodash.js

declare module "lodash" {
  declare module.exports: {
    find<T>(list: Array<T>, properties: Object): T;



flow-remove-types removes Flow type annotations from JavaScript files.

; npm install --global flow-remove-types
; flow-remove-types input.js > output.js

It also provides flow-node. Note that flow-node just removes type annotations and run JavaScript, it does not check types.


babel is slower than flow-remove-types, since it also transforms JavaScript to ES5.

Convert to JavaScript with babel:

; npm install -g babel-cli
; cd /path/to/my/project
; npm install babel-plugin-transform-flow-strip-types
; echo '{"plugins": ["transform-flow-strip-types"]}' > .babelrc
; babel --watch=./src --out-dir=./build

Run flow typed JavaScript file directly:

; babel-node /path/to/flow-typed.js

babel-node converts to JavaScript under the hood.


Webstorm is slow to inspect type errors. On my machine (16G RAM and SSD) it takes about 5s to inspect type errors with -Xms512m and -Xmx2048m. Disable “Code highlighting and built-in inspections” in settings will further reduce the time to 3s. Other settings (-Xms1024m/-Xmx4096m and -Xms256m/-Xmx1500m) is slower (about 10 seconds).

Nuclide and vscode (with vscode-flow-ide extension) checks error on saving file, which is instant. The features they provide for flow are similar to WebStorm.

Peek at the type system

Built-in types


JavaScript specifies many implicit conversions, which provide boolean semantics to values of other types. Flow allows this in conditional of if statements etc.

Note that in JavaScript Boolean(0) and new Boolean(0) are different. The formal is converting 0 to the boolean primitive type via the Boolean function, the later is initialize a Boolean wrapped object, which is rarely used. Same thing applies to number/Number and string/String.


JavaScript has a single number type, which is IEEE 754 floating point numbers.

Literal types

true, false, any number, and any string is a literal type.

let wrong: false = true && false is valid since a type checker can infer that for t typed true and f typed false, t && f is typed false, just like a type checker can infer that for m and n typed number, m + n is typed number. Unlike boolean, literal types of number and string has infinite members, a type checker cannot infer if the sum of two number typed as literal type has another specific literal type. So let two: 2 = 1 + 1 is invalid.

Included type definition

this type

A this type in Class C may be as the type of an instance of C, or a instance of a subtype of C. So this can only appear in a covariant position in the definition of C.

Function type

(p1: T1, .., pn: Tn) => U


Flow is pessimistic about refinements. If it is possible that a refinement may become invalid, Flow will throw away the refinement.

function foo(x: { y: ?string }): string {
  if (x.y) {
    console.log("*obviously* this doesn't mutate x");
    return x.y; // error: Flow doesn't know that
  } else {
    return "default";

So immutability is important. If x refers to an immutable record, Flow will be sure it will not be changed by console.log.



Just ues ES2015 or CommonJS module, with all export declaration type annotated.

CSS module


/* @flow */
declare export var className: string


module.name_mapper='^\(.*\)\.css$' -> '<PROJECT_ROOT>/CSSModule.js.flow'

<PROJECT_ROOT> will be replaced by Flow automatically.


/* @flow */

import {className} from "./SomeCSSFile.css"

Import type

export type UserID = number
export type User = {
  id: UserID,
  name: string,


import type {UserID, User} from "./User.js"

Declaration files

Flow’s declaration files are like C’s header files.

A declaration file of foo.js is named foo.js.flow:

/* @flow */

declare export function isLeapYear(year: string): boolean;
declare export let DEBUG: boolean;

declare export class Counter {
  val: number;
  increase(): void;

declare export type Response = 'yes' | 'no' | 'maybe';

declare export interface Stack<T> {
  push(item: T): void;
  pop(): T;
  isEmpty(): bool;

Declaration files supports mixin, which can also be used in implementations, for example,

/* @flow */

declare class MyClass extends Child mixins MixinA, MixinB {}
declare class MixinA {
  a: number;
  b: number;
// Mixing in MixinB will NOT mix in MixinBase
declare class MixinB extends MixinBase {}
declare class MixinBase {
  c: number;
declare class Child extends Base {
  a: string;
  c: string;
declare class Base {
  b: string;

var c = new MyClass();
(c.a: number); // Both Child and MixinA provide `a`, so MixinA wins
(c.b: number); // The same principle holds for `b`, which Child inherits
(c.c: string); // mixins does not copy inherited properties,
               // so `c` comes from Child


Interfaces use structural typing.

Go’s FAQ suggests to add an additional method to doc implementations:

type Fooer interface {

This is a simple workaround for documentation and it prevents accidental implementation of interfaces. However, it does not prevent a later modification of implementation of Fooer to remove Foo() and make ImplementsFooer() not correct any more.

With JavaScript, a much better workaround is possible (mentioned in #2376):

/* @flow */
interface A {
  foo(): number;

class AImpl {
  constructor(): A {
    return this;
  foo(): number {
    return 1

So flow’s structural typed interface is not as hopeless bad as Go.

Tuples/Arrays and objects are structural typed.

Objects has alternative nominal form called exact object type {|k: T|}. Prefer it.

Also prefer exact object type to tuple, i.e. {|0: J, 1: K|} instead of [J, K].

Parameters of functions may be viewed as tuples, they are structural too. Flow doc mentioned a workaround to prevent call a function with more parameters than defined:

function takesOnlyOneNumber(x: number, …rest: Array) {}

However, flow still does not check array bound (you have to check index against array length yourself).

Implicitly type casting number to string with +.

The number + string idiom is fairly common, so flow accepts it, like TypeScript.

With this patch, flow will reject number + string:

Error: main.js:3
  3:    return 1 + "s"
               ^ number. This type cannot be added to
  3:    return 1 + "s"
                   ^^^ string