Both TypeScript and Flow are very similar products and they share most of their syntax with some important differences. In this document I've tried to compile the list of differences and similarities between Flowtype and TypeScript -- specifically the syntax, usage and usability.
This document might be incomplete and/or contain mistakes and was last updated to describe TypeScript 2.6 and Flow 0.65.
I'm maintaining it in my spare time, so if you find mistakes, or learn about latest additions to either project, please help keep this repo up-to-date by contributing and editing this page.
Thanks!
TypeScript | Flow | |
---|---|---|
Leading Design Goal / North Star | identify errors in programs through a balance between correctness and productivity | enforce type soundness / safety |
IDE integrations | top-notch | sketchy, must save file to run type-check; some IDEs have workarounds to run real-time |
type-checking speed (w/o transpilation, subjective, need benchmarks!) | speed does not degrade much as the project grows | speed degrades with each additional file ![]() |
autocomplete |
|
|
expressiveness | great (since TS @ 2.1) | great |
type safety | very good (7 / 10) | great (8 / 10) |
specifying generic parameters during call-time | yes e.g. | no |
specifying generic parameters for type definitions | yes | yes |
typings for public libraries | plenty of well maintained typings | a handful of mostly incomplete typings |
unique features |
|
|
type spread operator | work in progress | shipped >=0.42 |
userland plugins | basic, not effecting emitting yet (planned) | no |
programmatic hooking | architecture prepared, work in progress | work in progress |
documentation and resources |
|
|
error quality | good | good in some, vague in other cases |
transparency | meeting notes, leadership reasoning and roadmap happens mostly publicly | low transparency, roadmap developed behind closed doors |
commercial support | no | no |
function fooGood<T: { x: number }>(obj: T): T {
console.log(Math.abs(obj.x));
return obj;
}
function fooGood<T extends { x: number }>(obj: T): T {
console.log(Math.abs(obj.x));
return obj;
}
https://flow.org/blog/2015/03/12/Bounded-Polymorphism/
let a: ?string
// equvalent to:
let a: string | null | void
let a: string | null | undefined
Optional parameters implicitly add undefined
:
function f(x?: number) { }
// same as:
function f(x?: number | undefined) { }
(1 + 1 : number);
(1 + 1) as number;
// OR (old version, not recommended):
<number> (1 + 1);
.flowconfig
[options]
module.name_mapper='^\(.*\)\.css$' -> '<PROJECT_ROOT>/CSSModule.js.flow'
CSSModule.js.flow
// @flow
// CSS modules have a `className` export which is a string
declare export var className: string;
declare module "*.css" {
export const className: string;
}
By default objects in Flow are not exact (can contain more properties than declared), whereas in TypeScript they are always exact (must contain only declared properties).
When using flow, { name: string }
only means “an object with at least a name property”.
type ExactUser = {| name: string, age: number |};
type User = { name: string, age: number };
type OptionalUser = $Shape<User>; // all properties become optional
TypeScript is more strict here, in that if you want to use a property which is not declared, you must explicitly say so by defining the indexed property. It is possible to use dotted syntax to access indexed properties since TypeScript 2.2. This is mostly a design decision as it forces you to write the typings upfront.
type ExactUser = { name: string, age: number };
type User = { name: string, age: number, [otherProperty: string]: any };
type OptionalUser = Partial<{ name: string, age: number }>; // all properties become optional
import type {UserID, User} from "./User.js";
TypeScript does not treat Types in any special way when importing.
import {UserID, User} from "./User.js";
Works the same in both cases, however Flow has an additional syntax to directly import a typeof
:
import typeof {jimiguitar as GuitarT} from "./User";
// OR (below also works in TypeScript)
import {jimiguitar} from "./User.js";
type GuitarT = typeof jimiguitar;
import {jimiguitar} from "./User";
type GuitarT = typeof jimiguitar;
class Test {};
type TestType = Class<Test>;
// This should be equivalent to (if you can confirm, please send a PR):
type TestType = typeof Test;
class Test {};
type TestType = typeof Test;
var props = {
foo: 1,
bar: 'two',
baz: 'three',
}
type PropsType = typeof props;
type KeysOfProps = $Enum<PropsType>;
function getProp<T>(key: KeysOfProps): T {
return props[key]
}
var props = {
foo: 1,
bar: 'two',
baz: 'three',
}
type PropsType = typeof props
type KeysOfProps = keyof PropsType;
function getProp<T>(key: KeysOfProps): T {
return props[key]
}
type $Record<T, U> = {[key: $Enum<T>]: U}
type SomeRecord = $Record<{ a: number }, string>
type SomeRecord = Record<{ a: number }, string>
type A = {
thing: string
}
// when the property is a string contant use $PropertyType (i.e. you know it when typing)
type lookedUpThing = $PropertyType<A, 'thing'>
// when you want the property to be dynamic use $ElementType (since Flow 0.49)
function getProperty<T : Object, Key : string>(obj: T, key: Key): $ElementType<T, Key> {
return obj[key];
}
Reference:
- facebook/flow#2952 (comment)
- https://github.com/facebook/flow/commit/968210c5887b5bdd47d17167300033d1e1077d1a
- facebook/flow#2464 (comment)
- flow/try
Arguably, it's a bit easier to type both cases in TS, since they follow the same pattern.
type A = {
thing: string
}
type lookedUpThing = A['thing']
// and...
function getProperty<T, K extends keyof T>(obj: T, key: K) {
return obj[key]; // Inferred type is T[K]
}
function setProperty<T, K extends keyof T>(obj: T, key: K, value: T[K]) {
obj[key] = value;
}
Reference:
Note: undocumented syntax, may change:
function isNil(value: mixed): boolean %checks {
return value == null;
}
const thing = null;
if (!isNil(thing)) {
const another = thing.something;
}
Reference:
Type-narrowing funtions are called type guard functions in TypeScript.
function isNil<T>(value: T | null): value is null {
return value == null;
}
const thing: any = null;
if (!isNil(thing)) {
const another = thing.something;
}
$Call
utility type:
type Fn1 = <T>(T) => T;
type E = $Call<Fn1, number>;
declare var e: E; // E is number
(42: E); // OK
Reference: https://github.com/facebook/flow/commit/ac7d9ac68acc555973d495f0a3f1f97758eeedb4
type InputType = { hello: string };
type MappedType = $ObjMap<InputType, ()=>number>;
Reference:
- https://gist.github.com/gabro/bb83ed574690645053b815da2082b937
- https://twitter.com/andreypopp/status/782192355206135808
A bit more flexibility here, as you have access to each individual key name and can combine with Lookup types and even do simple transformations.
type InputType = { hello: string };
type MappedType = {
[P in keyof InputType]: number;
};
It is possible to declare multiple signatures for the same method (also called: overloading). This feature is undocumented, and only available in type declarations (.js.flow
files or module statements), not inline/alongside your code.
declare function add(x: string, y: string): string;
declare function add(x: number, y: number): number;
declare class Adder {
add(x: string, y: string): string;
add(x: number, y: number): number;
}
However, it's possible to create function overloads inline for functions outside of classes, by using additional declarations.
declare function add(x: string, y: string): string;
declare function add(x: number, y: number): number;
function add(x, y) {
return x + y;
}
add(1, 1); // Ok
add("1", "1"); // Ok
add(1, "1"); // Error
TypeScript supports both function and method overloading, in both: type definitions (.d.ts
) and inline alongside code.
class Adder {
add(x: string, y: string): string;
add(x: number, y: number): number;
add(x, y) {
return x + y;
}
}
function add(x: string, y: string): string;
function add(x: number, y: number): number;
function add(x, y) {
return x + y;
}
type A = {
+b: string
}
let a: A = { b: 'something' }
a.b = 'something-else'; // ERROR
type A = {
readonly b: string
}
let a: A = { b: 'something' }
a.b = 'something-else'; // ERROR
One caveat that makes TypeScript's readonly
less safe is that the same non-readonly
property in a type is compatible with a readonly
property. This essentially means that you can pass an object with readonly
properties to a function which expects non-readonly properties and TypeScript will not throw errors: example.
empty
function returnsImpossible() {
throw new Error();
}
// type of returnsImpossible() is 'empty'
never
function returnsImpossible() {
throw new Error();
}
// type of returnsImpossible() is 'never'
Most of the syntax of Flow and TypeScript is the same. TypeScript is more expressive for certain use-cases (advanced mapped types with keysof, readonly properties), and Flow is more expressive for others (e.g. $Diff
).
function(a?: string) {}
In TypeScript, you can create more complex behaviors, like this:
function makeTgenerator<T>() {
return function(next : () => T) {
const something = next();
return something;
}
}
const usage = makeTgenerator<string>()
// 'usage' is of type: (next: () => string) => string
In Flow it is possible to define generic functions similarly to the above example, but only if one of the parameters or its return type is inferrable to the desired generic type, i.e. you cannot call any method/constructor using a custom T
.
function something(this: { hello: string }, firstArg: string) {
return this.hello + firstArg;
}
class SomeClass {
constructor(public prop: string, private prop2: string) {
// transpiles to:
// this.prop = prop;
// this.prop2 = prop2;
}
private prop3: string;
}
Add !
to signify we know an object is non-null.
// Compiled with --strictNullChecks
function validateEntity(e?: Entity) {
// Throw exception if e is null or invalid entity
}
function processEntity(e?: Entity) {
validateEntity(e);
let s = e!.name; // Assert that e is non-null and access name
}
type C = $Diff<{ a: string, b: number }, { a: string }>
// C is { b: number}
Note however that $Diff is not an official feature.
It only works properly as lower bound, i.e. you can assign something to it, but can't use it after that.
(source]
TypeScript has a proposal for an equivalent.
*
as a type or a generic parameter signifies to the type-checker to infer the type if possible
Array<*>
TypeScript has a proposal for an equivalent (needs link).
https://flow.org/en/docs/lang/variance/
function getLength(o: {+p: ?string}): number {
return o.p ? o.p.length : 0;
}
Bivariance is among the design decisions driving TypeScript.
The TypeScript equivalent of the mixed
type is simply:
type mixed = {}
Reference: https://flow.org/en/docs/types/mixed/
- microsoft/TypeScript#1265
- Undocumented Flow modifiers facebook/flow#2464
- http://sitr.us/2015/05/31/advanced-features-in-flow.html