A simple promise implementation that meets the Promises/A+ spec.

promise.js

function cbSucc(val) {
  console.log("Got my value! Yay!", val);
}

function cbErr(val) {
  console.log("D'oh!", val);
} 

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

        if(state === 'resolved') {
          handler.onResolved(value);
        } else {
          handler.onRejected(value);
        }
  };
    
    return {
        resolve: function (_value) {
  
         if(state === 'resolved') {
            throw new Error("A promise can only be resolved once."); 
            } 
        
        value = _value;
          state = 'resolved';

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

        reject: function (reason) {
        value = reason;
          state = 'rejected';
         if(deferred) {
            handle(deferred);
         }
        },

        then: function (onResolved, onRejected) {
            handle({
          onResolved: onResolved,
          onRejected: onRejected
        });
        }
    }
};


var oneSecondLater = function () {
    var result = defer();
    setTimeout(function () {
        if (Math.random() < .5) {
            result.resolve(1);
        } else {
            result.reject("Can't provide one.");
        }
    }, 1000);
    return result;
};  


oneSecondLater().then(cbSucc,cbErr);