A minimalist implementation of a javascript promise

tiny Promise.js

// (c) copyright unscriptable.com / John Hann
// License MIT
// For more robust promises, see https://github.com/briancavalier/when.js.

function Promise () {
	this._thens = [];
}

Promise.prototype = {

	/* This is the "front end" API. */

	// then(onResolve, onReject): Code waiting for this promise uses the
	// then() method to be notified when the promise is complete. There
	// are two completion callbacks: onReject and onResolve. A more
	// robust promise implementation will also have an onProgress handler.
	then: function (onResolve, onReject) {
		// capture calls to then()
		this._thens.push({ resolve: onResolve, reject: onReject });
	},

	// Some promise implementations also have a cancel() front end API that
	// calls all of the onReject() callbacks (aka a "cancelable promise").
	// cancel: function (reason) {},

	/* This is the "back end" API. */

	// resolve(resolvedValue): The resolve() method is called when a promise
	// is resolved (duh). The resolved value (if any) is passed by the resolver
	// to this method. All waiting onResolve callbacks are called
	// and any future ones are, too, each being passed the resolved value.
	resolve: function (val) { this._complete('resolve', val); },

	// reject(exception): The reject() method is called when a promise cannot
	// be resolved. Typically, you'd pass an exception as the single parameter,
	// but any other argument, including none at all, is acceptable.
	// All waiting and all future onReject callbacks are called when reject()
	// is called and are passed the exception parameter.
	reject: function (ex) { this._complete('reject', ex); },

	// Some promises may have a progress handler. The back end API to signal a
	// progress "event" has a single parameter. The contents of this parameter
	// could be just about anything and is specific to your implementation.
	// progress: function (data) {},

	/* "Private" methods. */

	_complete: function (which, arg) {
		// switch over to sync then()
		this.then = which === 'resolve' ?
			function (resolve, reject) { resolve && resolve(arg); } :
			function (resolve, reject) { reject && reject(arg); };
		// disallow multiple calls to resolve or reject
		this.resolve = this.reject = 
			function () { throw new Error('Promise already completed.'); };
		// complete all waiting (async) then()s
		var aThen, i = 0;
		while (aThen = this._thens[i++]) { aThen[which] && aThen[which](arg); }
		delete this._thens;
	}
	
};

You've done a few wonderful things in this to keep the code incredibly short, and yet it is still very readable. Props, and thank you.

What about catch() for wrap errors throws?

Promises can only be resolved by function which has created it. How does it takes care of that.

function something(){
  var p = new Promise() ;
  setTimeout(function(){
    doSomeAsyncXhr().then(function(){
     p.resolve();
   });
  },5000);
 return p;
}

var p = something();
p.resolve();

Yours are more like a deferred promise...
So i wanted to try write a similar a+ promise and only try to do the bare minimum

This is what i ended up with when it was minified (hand crafted):

TinyPromise = (h, f = [], b = -1, g, c, l) => (
  l = d => {for (g = d; c = f.shift();) c[b] && c[b](d)},
  // execute the handler(resolve, reject)
  h(d => l(d, b = 0), d => l(d, b = 1)),
  // return a thenable (non chainable)
  {then(...d){ ~b ? d[b] && d[b](g) : f.push(d)} }
)

usage:

TinyPromise((resolve, reject) => resolve(3)).then(console.log, console.error)

Someone wants to try to beat this or try to make it a little better? 😜
Goal is not to make it complete spec compatible but to make it do just what you need

@jimmywarting I think it will be better to post the uncompressed version so you have better feedback about this

How about one that is chainable, that returns an actual instance ?

function P (cb) {
  var q = [], v, u, ok, complete = function (m, r) {
    if (q) {
      var i = -1, l = q;
      ok = !m; v = r; q = null;
      while (++i < l.length) l[i][m](v);
    }
  };

  cb(complete.bind(u, 0), complete.bind(u, 1));

  this.then = then;
  this.catch = then.bind(u, u)

  function then(success, error) {
    return new P(function (resolve, reject) {
      if (q) q.push([ done.bind(u, success), done.bind(u, error) ]);
      else done(ok ? success : error);

      function done (cb) { try {
        var val = cb ? cb(v) : u;
        if (val && val.then) val.then(resolve, reject);
        else (cb || ok ? resolve : reject)(val);
      } catch (e) { reject(e) } }
    })
  }
}

i guess a much more simpler implementation could be this

var Promise = function() {
    var this._callBacks = [];

    this.then = function(fn) {
        _callBacks.push(fn);
        return this;
    }

    this.resolve = function(data, err) {
        this._callBacks[0].call(this, data, err);
        this._callBacks.shift();
    }
}



function readPromise(filename) {
    var promise = new Promise();

    fs.readFile(filename, function(err, data) {
        promise.resolve(data, err);
    });
    return promise;
}

readPromise("./myfile.js")
    .then(function(data, err) {
        //-- do something here -- //
        //-- resolve promise with this.resolve(data, err) -- //
    })
    .then(function(data, err) {
        //-- do something here -- //
    });