Example 3-5. Caching a promise

var user = {
	loginPromise: null,

	login: function () {
		var me = this;
		if (this.loginPromise == null) {
			this.loginPromise = ajax(/*someurl*/);

			// Remove cached loginPromise when a failure occurs to allow retry
			this.loginPromise.catch(function () {
				me.loginPromise = null;
			});
		}
		return this.loginPromise;
	}
};