JavaScript signature notation
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jsig - precise & concise JavaScript signature notation

##Stability Note

This specification is a draft. The spirit of what we intend to capture is here, but the exact wording is still being ironed out. Expect change. Please participate with your comments and feedback.


JavaScript is a a dynamically typed language. Most of the time, this is great - this flexibility lets us write things like "I'm " + thisMany and have it work whether thisMany === 'five' or thisMany === 5. But sometimes it helps to have a vocabulary for explicitly stating expectations about the values and objects our code is expecting, especially when writing documentation for other humans.

JSDoc is an attempt to solve this, but falls short by limiting itself to built-in types and instanceof constructors. In particular, it is difficult to describe function signatures for callbacks and objects with a certain set of properties.

jsig is a system of notation using structure-based type annotations to describe JavaScript interfaces. It aims to be explicit, concise, and familiar for those used to JavaScript syntax.

The goal of this system of annotation is not necessarily to be computable but to aid communication between programmers.


// (array: Array, (accumulated: Any, elem: Any) => Any, seed: Any) => Any
function reduce(array, reducer, seed) {
  return array.reduce(reducer, seed);

Read through for further examples. Please contribute your own via a pull request.


jsig specifies values and describes their types. It can be used to describe a single value, for example a variable or a configuration object. It can also describe a function as a value, and its relationship between its parameters and a return value. This relationship is called the function's signature. It can also be used to create named types to be used in other signatures

Basic Types

The built-in JavaScript constructor functions describe basic types: String, Array, RegExp, etc. For example, in JavaScript we can describe a bicycle like so:

var bicycle = {
  gears: 10,
  tires: 'hydrid',
  color: 'white',
  lastUsed: new Date()

In jsig we can make statements about this object.

bicycle : {
  gears: Number,
  tires: String,
  color: String,
  lastUsed: Date

Generic Types

Some types of values can be described as generic, or "types of types." For example, the JavaScript value:

var cats = ['tabby', 'shorthair', 'calico', 'persian']

could be described as an "array of strings." In jsig this is expressed as Array<String>.

Formally, the generic type preceeds the specific type, which is surrounded by angle brackets <,>. This notation should be familiar to users of Java or C#. Here, we borrow the notation for the purpose of simplifying otherwise quite verbose notations. Consider Callbacks or Promises in asynchronous JavaScript programming.

In Node.js, we can read a file using fs.readFile. We could write this signature (simplified) as:

readFile : (
  filename: String,
  options?: Object,
  callback: (err: Error, contents: Buffer) => void
) => void

or we could create a generic Callback<Type> notation and write:

type Callback<T> : (err: Error, contents: T) => void

readFile : (
  filename: String,
  options?: Object,
  callback: Callback<Buffer>
) => void

Using Promise values, we could simplify and clarify the type signature even further to:

type Promise<T> : {
  then: ((result: T) => S, (error: Error) => S) => Promise<S>,
  catch: ((error: Error) => S) => Promise<S>

readFile (
  filename: String,
  options?: Object
) => Promise<Buffer>

Generic types should be fully specified as a Custom Type (see below). In the custom type definition, any symbol may be used inside the angle brackets and should be internally consistent in the definition. In this example, the definition for a generic Callback would be:

type Callback<T> : (err: Error, result: T) => void

Multiple generic types may be specified if necessary. Type parameters should be separated by a comma, e.g.:

Generic<Type1, Type2>

Generic objects

When dealing with a generic object you could express the following:

  oranges: { color: 'orange', cost: 1.2 },
  bananas: { color: 'yellow', cost: 0.8 },
  apples:  { color: 'red', cost: 1.5 }


Object<String, {
  color: String,
  cost: Number

Here we use Object<String, T> to say that the type of a value is an object with string keys whose values are of type T.

This means that the object is homogenous.

Note that since the type is Object<String, T> you are allowed to change the first argument to a nice alias. For example:

type FruitName : String

Object<FruitName, {
  color: String,
  cost: Number

Custom Types

When describing an object with a well-defined interface (for example, from a constructor function or an interface specification like Promises/A+), this name should appear in PascalCase and should refer either to the name of the constructor or should otherwise be obvious in context (eg, ReadStream or HttpClientRequest in Node.js or DOMElement in a browser).

// (req: HttpServerRequest, res: HttpServerResponse) => void
function endpoint(req, res) {

If possible you should either define or import any custom types you use.

import { HttpServerResponse } from 'jsig-node-types'

// subset of HttpServerRequest
type HttpServerRequest : {
  method: String,
  url: String

endpoint : (req: HttpServerRequest, res: HttpServerResponse) => void

Custom types can be named like function parameters using the type keyword and a colon character :, and should use PascalCase:

type User: { id: Number, name: String, email: String }

Special Types

Any Value

Values that may be of any type (or unspecified type) should be written as:


No Value

No value (for example, a function which does not return) should be specified using the JavaScript keyword void


Literal value

Literal values should be specified using their JavaScript literals, eg null, true, false, "a string", 22

Multiple Types

When a value may be of multiple types, the types may be joined by a pipe character | (read as logical "or"):

// (names: String|Array<String>) => void
function greet(names) {
  if (!Array.isArray(names)) {
    console.log('hey ' + names)
  } else {
    console.log('hey ' + names.join(', '))

Compound Types (or "extends", "mixins", etc)

When a value should have multiple types (eg, when combining multiple interfaces), these types may be joined by an ampersand character & (read as logical "and"):

Response: ReadStream & {statusCode: Number}

Compound types and Multiple types can use parenthesis to make associativity explicit, although for the sake of clarity it may be beneficial to use named CustomTypes or explicitly list each signature combination.

Function Signatures

Functions are specified using proposed ES6 arrow notation:

(param, param2) => Type

To specify any function, use the built-in constructor Function.

Return types must be specified.

Functions may be named by adding a label in front of the function definition. When written as a comment above the function definition, this is typically omitted.

getElementById: (String) => DOMElement


Parameters may be named for clarity and documentation purposes. The parameter name is separated from the type by a colon. Whitespace is ignored. The parameter name is optional, but a type must be specified.

The return value can also be labeled.

// anonymous signature
(String, Number) => Boolean

// named signature
isMinLength: (str: String, min: Number) => isValid: Boolean

Variadic (variable number of parameters) functions

When a function accepts an arbitrary number of parameters of the same type, they may be specified with the ES6 Rest Parameter syntax, consisting of three periods before the parameter name:

// (name: String, ...pets: String) => Any
function petOwner(name) {
  var pets =, 1)
  console.log(name + ' owns ' + pets.join(', '))

Return types

Functions should return a single type, indicated after the arrow notation, eg () => Type.

If a function returns different types depending on the parameters, this should be explicitly noted and each function signature should be fully specified:

fooAsync: (callback: Function) => void &
    () => Promise

Note that we can use a compound type to specify that fooAsync supports both the callback interface and the promise interface.

This is similar to method overloading in static languages.

Functions which do not return a type should be specified as () => void


When describing a function's parameters list, an optional parameter can be specified by appending a question mark ? to the parameter name:

// (requestUrl: String, timeout?: Number) => Promise<String>
function get(requestUrl, timeout) {
  // do stuff


Arrays with a specific number of typed elements can be specified using JavaScript array literal notation with types matching the element indices:

point: [x: Number, y: Number, z: Number]

Structure Objects

An object with certain properties is expressed as a JavaScript object literal with property names corresponding to the names on the expected object:

// ({stderr: ReadStream, stdout: ReadStream }) => void
function log(stdio) {
  file = require('fs').createWriteStream('io.log')

Optional properties can be indicated by a ? following the name:

// (user: {id: Number, email?: String}) => Boolean
function hasEmail(user) {
  return user.hasOwnProperty('email')


jden raynos


Public Domain.


To the extent possible under law, all copyright and related or neighboring rights to this work have been waived. This work is published from: United States.