Chapter 14, page 289, in the section Promise-ifying the other DataStore methods
The last code listing on the page shows what code to remove from DataStore.prototype.add.
The line this.data[key] = val; should not be removed.
The final version of DataStore.prototype.add should be as follows:
DataStore.prototype.add = function (key, val) {
this.data[key] = val;
return promiseResolvedWith(null);
};