autocreate

autocreate

Sometimes, you need something to be both a class and a function. Sometimes, you need a class to be callable without using new. The autocreate module makes both as easy as typing @auto before your class declaration (in Babel/ES7 or TypeScript), and almost that easy in CoffeeScript or plain Javascript.

auto = require('autocreate');

@auto class MyClass {
  constructor() {
    // No need to check for `this instanceof`;
    // just do whatever needs doing.
  }
}

autocreate also includes a special hook method that you can define to customize what happens when the class is called without new. That way, you can return an object from a cache, return an object that was passed in (like Object(obj) does), or generaly do things differently when invoked without new:

@auto class dualUse {
  constructor() {
    // this is called if you use `new dualUse()`
  }
  __class_call__() {
    // this is called if you use `dualUse()`
  }
}

For more usage details, including code samples for all of the supported languages, see the "Usage" section, below. (autocreate will also probably work with most other compile-to-Javascript languages; read the "Implementation Details" section for more info!)

autocreate supports even the most exotic class features of its supported languages, including static methods and properties, automatically bound methods, etc. -- even non-enumerable property descriptors and getter/setters.

It does not, however, depend on any particular version of Javascript as its execution environment: if you're using a language whose features work on old versions of IE, then autocreate will work there too! If you're targeting an ES5 or ES6 environment, no problem: autocreate will detect the relevant features (descriptors, symbols, __proto__, etc.) and progressively enhance itself to support them.

Contents

• Usage
  • Create a new-less class
  • Create a class with separate "function" behavior
• Implementation Details
  • Constructor Wrapping
  • In-Namespace Replacement
  • Return Value Handling
  • Inheritance

Usage

Create a new-less class

In TypeScript (or Babel w/ES7 features enabled)

auto = require('autocreate');

@auto class MyClass {
  constructor() {
    // No need to check for `this instanceof`;
    // just do whatever needs doing.
  }
}

In CoffeeScript

auto = require 'autocreate'

class MyClass

  # Because CoffeeScript doesn't have decorators, you have to
  # assign to the class name *inside* the class body
  
  MyClass = auto @

  constructor: ->
    # no need to check `this instanceof`;
    # just do whatever needs doing

In Plain Javascript

var auto = require('autocreate');

MyClass = auto(MyClass);

function MyClass () {
  // No need to check for `this instanceof`;
  // just do whatever needs doing.
});

Create a class with separate "function" behavior

In TypeScript (or Babel w/ES7 features enabled)

@auto class MyClass {
  constructor() {
    // This will run if `new MyClass()` is used
  }
  __class_call__() {
    // This will run if `MyClass()` is called:
    // `this` will be `MyClass.prototype`
  }
}

In CoffeeScript

class MyClass
  MyClass = auto @

  constructor: ->
    # This will run if `new MyClass()` is used

  __class_call__: ->
    # This will run if `MyClass()` is called:
    # `this` will be `MyClass.prototype`

In Plain Javascript

MyClass = auto(MyClass);

function MyClass () {
  // This will run if `new MyClass()` is used
});

MyClass.prototype.__class_call__ = function () {
  // This will run if `MyClass()` is called:
  // `this` will be  `MyClass.prototype`
}

Implementation Details

Constructor Wrapping

autocreate works by creating a new constructor function to wrap the old one. It copies any static methods, properties, descriptors, __proto__ etc., so that even Babel's most advanced class features will still work on the result. It even shares the original constructor's .prototype and resets the .constructor, so that created objects will still be an instanceof both the wrapper and the wrapped constructor.

In-Namespace Replacement

For this to work correctly in a given language, the wrapper must replace the wrapped constructor in the same namespace where it was defined.

For plain Javascript, this is simple: just set MyClass = auto(MyClass);, and you're done. For other languages, it's a little more complex, because most compile-to-Javascript languages wrap the definition of a class constructor inside a closure.

For TypeScript and Babel (w/ES7 decorators enabled), the @auto decorator syntax handles this: it actually replaces the constructor inside the closure, in a way that would not always work if you just used MyClass = auto(MyClass);. (Because generated methods inside the class-closure could refer to the wrong constructor.)

CoffeeScript, however, doesn't have a decorator syntax, which is why you have to do MyClass = auto @ (or MyClass = auto this) inside the class body. If you do it from outside the class body, all the code inside the class body will refer to the original constructor!

Also, it's important to note that the MyClass part of the statement must match CoffeeScript's generated class name. That is:

class MyClass
  MyClass = auto @      # default

class foo.bar.Baz
  Baz = auto @          # last .name
  
class foo('Bar')['Baz'].Spam
  Spam = auto @         # even with expressions!

class
  _Class = auto @       # an anonymous class

class foo('Bar')['Baz']
  _Class = auto @       # no last .name

Determining the correct way to do this in the language of your choice (other than Babel, TypeScript, CoffeeScript, etc.) is left as an exercise for the reader. (Take a look at the code your language generates for a class statement, or if it has an online interactive "playground" the way Babel, TS, and CS do, even better!)

Return Value Handling

autocreate respects the standard Javascript rules for constructor return values. If your wrapped constructor returns an object or function, it will be returned in place of the newly-created instance -- regardless of whether the wrapper was called with new or not.

On the other hand, if you supply a __class_call__ method, its return value will always be returned from the wrapper, regardless of type. This allows the class to behave like a normal function, when it's called as one.

(Of course, if you want your __class_call__ to return an instance, you can create one via new this.constructor(...), because this in a __class_call__ is equal to the class's .prototype.)

Inheritance

If you create a subclass of an autocreate class, you should make it autocreate as well. This is because it's impossible for a constructor to know whether it's being called directly, or via a subclass super call. Thus, even though the base class can tell it wasn't invoked with new, it can't tell which class it should create an instance of!

Also, you should be aware that since __class_call__ is a normal instance method, it is automatically inherited by subclasses. If you don't want the subclasses to respond to it, you can override the method in the subclasses, or you can write the method like this (Babel/ES7):

@auto class BaseClass {
  __class_call__() {
    if (this !== BaseClass.prototype) {
      // Create and return an instance of the correct subclass
      return new this.constructor(...arguments)
    }
    // ... 
    // whatever `BaseClass()` should do w/out `new`
  }
}

@auto class Subclass extends BaseClass {
  // ... etc.
}

This will then return a Subclass instance when you call Subclass() without new, but fall through to whatever special handling you've set up when you call BaseClass() without new.