How To Avoid Manually Caching A Request That Returned An Error?

I've created a module (following Javascript's Module Pattern) that makes an http request, caches and returns the result: var requestService = (function($) { var cache = {};

Solution 1:

done and fail do not support chaining callback results, your return new Promise is absolutely pointless. Also it looks as if you are trying to use the Promise constructor antipattern. What your code does it to simply return the ajax promise as-is (which works, as you can chain onto it).

the results are cached even if there's an error

Yes - by storing the promise, the whole request result is cached, not only in the success case.

Should I add a rejection handler, setting itself to null?

Yes, that's exactly what you'll need to do:

var requestService = (function($) {
    var cache = {};

    functionget(date) {
        var params = date ? {'date': date} : {};
        return $.getJSON('http://my/url', params);
    }

    functionfetchData(date) {
        if (!cache[date]) {
            cache[date] = get(date);
            cache[date].fail(function(err) {
                cache[date] = null; // retry nexttime
            });
        }
        return cache[date];
    }

    return {
        fetchData: fetchData
    };
})(jQuery);

If you want to return a native promise, use

functionfetchData(date) {
    if (!cache[date]) {
        cache[date] = Promise.resolve(get(date)).catch(function(err) {
            cache[date] = null; // retry nexttime
            throw err;
        });
    }
    return cache[date];
}

Solution 2:

ok nevermind, that code worked with some adjustments. In my fetchData method I'm checking if the object is empty (so undefined or null) and I've added a fail() method that nullifies the cache[date]. I do the following now:

var requestService = (function($) {

    var cache = {};

    var get = function(date) {
        var params = date ? {'date': date} : {};
        return $.getJSON('http://my/url', params, function( result ){});
    };

    var fetchData = function(date) {
        // now also checks for nullif ($.isEmptyObject(cache[date])) {
            cache[date] = get(date);
        }

        return cache[date].done(function(myData) {
            returnnewPromise(function(resolve,reject) {
                resolve(myData);
            });
        }).fail(function(myData) {
        returnnewPromise(function(resolve,reject) {
            // nullifies cache[date]
            cache[date] = null;
            reject(myData);
        });
    };

    return {
        fetchData: fetchData
    };
})(jQuery);