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A flow compatible type system for JS.




Flow Runtime

A runtime type system for JavaScript with full Flow compatibility.


Provides a rich API for defining, inspecting and verifying data types in JavaScript. Any value that can be represented in JS can be represented by flow-runtime, including full support for polymorphism and parameterized types.

See the docs for more information.


import t from 'flow-runtime';

const number = t.number();
const string = t.string();

string.accepts('foo'); // true
string.accepts(123); // false
number.accepts(123); // true

string.assert('Hello World!'); // ok
string.assert(false); // throws
number.assert(456); // ok
number.assert('nope'); // throws

const numberOrString = t.union(number, string);

numberOrString.assert(123); // ok
numberOrString.assert("baz"); // ok
numberOrString.assert(false); // throws

const fooOrBar = t.union(

fooOrBar.assert('foo'); // ok
fooOrBar.assert('bar'); // ok
fooOrBar.assert('qux'); // throws

const Thing = t.object('name', t.string()),'url', t.nullable(t.string()))

  name: 'Example',
  url: ''
}); // OK

  name: 'Example'
}); // OK

  name: false
}); // throws

const arrayOfStrings = t.array(t.string());


// ---------------------------------------------

const UserStatus = t.union(

const PreferenceName = t.union(

const UserPreferences = t.object(
  t.indexer(PreferenceName, t.boolean())

const User = t.object({
  id: t.number(),
  name: t.string(),
  email: t.string(),
  status: UserStatus,
  preferences: UserPreferences

const validUser = {
  id: 123,
  name: 'Sally',
  email: '[email protected]',
  status: 'PENDING',
  preferences: {
    marketingOptIn: true

const invalidUser = {
  id: false, // invalid
  name: 'Bob',
  email: '[email protected]',
  status: 'NOPE', // invalid
  preferences: {
    marketingOptIn: true,
    nope: true // invalid

User.accepts(validUser); // true
User.accepts(invalidUser); // false

User.assert(validUser); // OK
User.assert(invalidUser); // throws TypeError