30 days of code promise implementation from http://mattgreer.org/articles/promises-in-wicked-detail/

promise.js

/** 30_days_of_code
* Playing with promise implementations
*
*  used doc here to step by step build: http://mattgreer.org/articles/promises-in-wicked-detail/
*/

function Promise(fn) {
  var state = 'pending';
  var value;
  var deferred = null;

  function resolve(newValue){
    try {
      //only looking for .then method not our own Promise implementation
      //helps if you are intermingling promise libraries
      if(newValue && typeof newValue.then === 'function'){
        newValue.then(resolve, reject);
        return;
      }

      value = newValue;
      state = 'resolved';

      if(deferred){
        handle(deferred);
      }
    } catch(e){
      reject(e);
    }
  }

  function reject(reason){
    state = 'rejected';
    value = reason;

    if(deferred){
      handle(deferred);
    }
  }

  function handle(handler){
    if(state==='pending'){
      deferred == handler;
      return;
    }

    var handlerCallback;

    if(state==='resolved'){
      handlerCallback = handler.onResolved;
    }else{
      handlerCallback = handler.onRejected;
    }

    if(!handlerCallback){
      if(state==='resolved'){
        handler.resolve(value);
      }else{
        handler.reject(value);
      }

      return;
    }

    var ret;
    try {
      ret = handlerCallback(value);
    } catch(e) {
      console.warn('caught an unexpected error');
      handler.reject(e);
      return;
    }

    handler.resolve(ret);
  }

  this.then = function(onResolved, onRejected){
    return new Promise(function(resolve, reject){
      handle({
        onResolved: onResolved,
        onRejected: onRejected,
        resolve: resolve,
        reject: reject
      });
    });
  };

  fn(resolve, reject);
}

function doSomething() {
  return new Promise(function(resolve, reject){
    var val = Math.floor(Math.random() * (10 - 1 + 1)) + 1;
    console.log('Original Value:', val)

    if(val>=6){
      console.log('value to high reject!');
      reject(val);
    }else{
      resolve(val);
    }
  });
}

function rejector() {
  return new Promise(function(resolve, reject){
    var val = Math.floor(Math.random() * (10 - 1 + 1)) + 1;

    reject(val);
  });
}

function nonRejector() {
  return new Promise(function(resolve, reject){
    var val = Math.floor(Math.random() * (10 - 1 + 1)) + 1;

    resolve(val);
  });
}

function err(err){
  console.log(err);
}

var p = doSomething;
var values = [];
p()
.then(function(val){
  
  values = [val];
  console.log('1:', val, values);
  
  return val + 22;
}, function(err){console.log('error', err)})

.then(function(val){
  values.push(val);
  console.log('2:', val, values);
  
  //if we return it promis it will now wait for this
  //return doSomething();
  return val;

}, function(err){console.log('error', err)})
.then(function(val){
  values.push(val);
  console.log('3:', val, values);
},function(err){console.log('error', err)});



var d = rejector;

d().then(function(val){
  console.log(val);
}, function(err){
  console.log('err', err);
})

var d = nonRejector;

d().then(function(val){
  console.log(val);
  throw err;
}, function(err){
  console.log('err', err);
})