A quick promise implementation in JS to showcase how simple it is to implement a promise. Don't use in a production app!

promise.js

function promise() {
  var listeners = [],
      resolved = false,
      data;

  return {
    isResolved: function() {
      return resolved;
    },
    
    whenDone: function(cb) {
      if(resolved)
        cb(data);
      else 
        listeners.push(cb);
    },
    
    resolve: function(d) {
      resolved = true;
      data = d;
      
      listeners.forEach(function(cb) {
        cb(data);
      });
      
      listeners = [];
    }
  };
}



// Sample usage
var p = promise();

//...

p.whenDone(function(data) {
  // do something with the data
});

// ...

p.resolve(42);