Sample Promise Implementation. Wrote in https://coderpad.io/626322

promise.js

// --------------------------------------------- Promise Implementation
Promise = function () {
  this._stack = [];
  this._isResolved = false;
}
Promise.prototype = {
  success: function(callback){
    // Is the promise already resolved?
    if(this._isResolved) {
      callback( this._result );
    } else {
      this._stack.push(callback);
    }
  },
  resolve: function(result){
    if(this._isResolved) { throw new Error('Promise Has Already Resolved'); }
    this._result = result;
    // Clear the promise stack
    for (var i = 0; i < this._stack.length; i++) {
      this._stack[i](result);
    }
    this._isResolved = true
  }
}




// --------------------------------------------- "Assertions"

var foo = new Promise();
var bar = new Promise();

foo.resolve('hello');

setTimeout(function(){
  bar.resolve('world');
}, 500);

foo.success(function(result){
  console.log(result);
});

bar.success(function(result){
  console.log(result);
});

// => "hello"
// ... 500ms later ...
// => "world"


//A promise should only be able to be resolved once. Subsequent calls to resolve should error.
setTimeout(function(){
  bar.resolve('world again'); // Throws error
}, 750);

//A promise may have multiple success callbacks attatched.
setTimeout(function(){
  var baz = new Promise();
  baz.success(function(result){ console.log(result); });
  baz.success(function(result){ console.log(result.split('').reverse().join('')); });
  baz.resolve('Rainbow Bunny!');
}, 1000);