Caching a jquery ajax response in javascript/browser

cache:true only works with GET and HEAD request.

You could roll your own solution as you said with something along these lines :

var localCache = {
    data: {},
    remove: function (url) {
        delete localCache.data[url];
    },
    exist: function (url) {
        return localCache.data.hasOwnProperty(url) && localCache.data[url] !== null;
    },
    get: function (url) {
        console.log('Getting in cache for url' + url);
        return localCache.data[url];
    },
    set: function (url, cachedData, callback) {
        localCache.remove(url);
        localCache.data[url] = cachedData;
        if ($.isFunction(callback)) callback(cachedData);
    }
};

$(function () {
    var url="/echo/jsonp/";
    $('#ajaxButton').click(function (e) {
        $.ajax({
            url: url,
            data: {
                test: 'value'
            },
            cache: true,
            beforeSend: function () {
                if (localCache.exist(url)) {
                    doSomething(localCache.get(url));
                    return false;
                }
                return true;
            },
            complete: function (jqXHR, textStatus) {
                localCache.set(url, jqXHR, doSomething);
            }
        });
    });
});

function doSomething(data) {
    console.log(data);
}

Working fiddle here

EDIT: as this post becomes popular, here is an even better answer for those who want to manage timeout cache and you also don’t have to bother with all the mess in the $.ajax() as I use $.ajaxPrefilter(). Now just setting {cache: true} is enough to handle the cache correctly :

var localCache = {
    /**
     * timeout for cache in millis
     * @type {number}
     */
    timeout: 30000,
    /** 
     * @type {{_: number, data: {}}}
     **/
    data: {},
    remove: function (url) {
        delete localCache.data[url];
    },
    exist: function (url) {
        return !!localCache.data[url] && ((new Date().getTime() - localCache.data[url]._) < localCache.timeout);
    },
    get: function (url) {
        console.log('Getting in cache for url' + url);
        return localCache.data[url].data;
    },
    set: function (url, cachedData, callback) {
        localCache.remove(url);
        localCache.data[url] = {
            _: new Date().getTime(),
            data: cachedData
        };
        if ($.isFunction(callback)) callback(cachedData);
    }
};

$.ajaxPrefilter(function (options, originalOptions, jqXHR) {
    if (options.cache) {
        var complete = originalOptions.complete || $.noop,
            url = originalOptions.url;
        //remove jQuery cache as we have our own localCache
        options.cache = false;
        options.beforeSend = function () {
            if (localCache.exist(url)) {
                complete(localCache.get(url));
                return false;
            }
            return true;
        };
        options.complete = function (data, textStatus) {
            localCache.set(url, data, complete);
        };
    }
});

$(function () {
    var url="/echo/jsonp/";
    $('#ajaxButton').click(function (e) {
        $.ajax({
            url: url,
            data: {
                test: 'value'
            },
            cache: true,
            complete: doSomething
        });
    });
});

function doSomething(data) {
    console.log(data);
}

And the fiddle here CAREFUL, not working with $.Deferred

Here is a working but flawed implementation working with deferred:

var localCache = {
    /**
     * timeout for cache in millis
     * @type {number}
     */
    timeout: 30000,
    /** 
     * @type {{_: number, data: {}}}
     **/
    data: {},
    remove: function (url) {
        delete localCache.data[url];
    },
    exist: function (url) {
        return !!localCache.data[url] && ((new Date().getTime() - localCache.data[url]._) < localCache.timeout);
    },
    get: function (url) {
        console.log('Getting in cache for url' + url);
        return localCache.data[url].data;
    },
    set: function (url, cachedData, callback) {
        localCache.remove(url);
        localCache.data[url] = {
            _: new Date().getTime(),
            data: cachedData
        };
        if ($.isFunction(callback)) callback(cachedData);
    }
};

$.ajaxPrefilter(function (options, originalOptions, jqXHR) {
    if (options.cache) {
        //Here is our identifier for the cache. Maybe have a better, safer ID (it depends on the object string representation here) ?
        // on $.ajax call we could also set an ID in originalOptions
        var id = originalOptions.url+ JSON.stringify(originalOptions.data);
        options.cache = false;
        options.beforeSend = function () {
            if (!localCache.exist(id)) {
                jqXHR.promise().done(function (data, textStatus) {
                    localCache.set(id, data);
                });
            }
            return true;
        };

    }
});

$.ajaxTransport("+*", function (options, originalOptions, jqXHR, headers, completeCallback) {

    //same here, careful because options.url has already been through jQuery processing
    var id = originalOptions.url+ JSON.stringify(originalOptions.data);

    options.cache = false;

    if (localCache.exist(id)) {
        return {
            send: function (headers, completeCallback) {
                completeCallback(200, "OK", localCache.get(id));
            },
            abort: function () {
                /* abort code, nothing needed here I guess... */
            }
        };
    }
});

$(function () {
    var url="/echo/jsonp/";
    $('#ajaxButton').click(function (e) {
        $.ajax({
            url: url,
            data: {
                test: 'value'
            },
            cache: true
        }).done(function (data, status, jq) {
            console.debug({
                data: data,
                status: status,
                jqXHR: jq
            });
        });
    });
});

Fiddle HERE
Some issues, our cache ID is dependent of the json2 lib JSON object representation.

Use Console view (F12) or FireBug to view some logs generated by the cache.