I missed extensions from Swift / Objective-C, so I brought them to Javascript.

install

npm i adopt

use

const Extension = require('adopt').Extension

// Create an extension.
// An extension adds private methods to the types you specify.
// This extension adds a private toJson method to all Objects.
const jsonExt = Extension(Object) ({
  toJson(...args) {
    return JSON.stringify(this, ...args)
  }
})

const obj = {whee: 'this is fun'}

// You use brackets to call a private method, by
// looking it up in the extension.
//
// Like this:
console.log(obj [jsonExt.toJson] (null, 2))

// Prints:
//   {
//     "wheee": "this is fun"
//   }

[Extension.defaultKey]

We might prefer to do this instead, though:

const json = Extension(Object) ({
    // The default key is what you'll get if you use the extension itself
    // to look up a member in an Object.
    [Extension.defaultKey]: 'json',
    
    // Oh, also, getters and setters work fine.
    get json() { return JSON.stringify(this) },
    
    toJson(...args) { return JSON.stringify(this, ...args) }
})

const self = {
    brain: {thoughts: {distracting: 'many', curious: true}},
    heart: {isOpen: true}
}

// Isn't this nice?
console.log(self [json])

// And this still works.
console.log(self [json.toJson] (0, 2))

You can be more specific in what you extend, or extend multiple things.

const sample = Extension(Array, String) ({
    _: require('lodash'),
    sample(...args) {
        return this._sample(this, ...args)
    }
})

how does this work?

It monkey patches prototypes.

Every key in the object you provide gets mapped to a unique Symbol. We add new properties to every prototype with those symbols as keys, then we return the mapping.

i thought monkey patching Object.prototype was very bad?

It's bad with string keys, because there's no way to pick a name that you know won't be used in a future version of Javascript (unless you can see the future, in which case you do not need this library).

Symbols, introduced in ES6, make this safe.

Like Strings, Symbols can be keys for objects.

Unlike Strings, every Symbol is unique. Not only is collision unlikely, it is impossible.

the future

As the name suggests, I also miss protocols. Those are not yet implemented by this library, though perhaps the name is suggestive.