Implementing promises

function makePromise(callback){
  var onsuccess = function(){};
  var onerror = function(){};
  var promise = {
    then: function(func){
      onsuccess = func;
      return this;
    },
    catch: function(func){
      onerror = func;
      return this;
    }
  };
  function resolve(data){
    onsuccess.call(null, data);
  }
  function reject(err){
    onerror.call(null, err);
  }
  callback.call(null, resolve, reject);
  return promise;
}
makePromise(function(resolve, reject){
  setTimeout(function(){
    Math.random() > 0.5 ? resolve(42) : reject(new Error("An error has occurred!"));
  }, 500);
}).then(function(x){
  console.log(x);
}).catch(function(err){
  console.log(err);
});