caching JavaScript files

Which is the best method to make the browser use cached versions of js files (from the serverside)?

Have a look at Yahoo! tips: https://developer.yahoo.com/performance/rules.html#expires.

There are also tips by Google: https://developers.google.com/speed/docs/insights/LeverageBrowserCaching

or in the .htaccess file

AddOutputFilter DEFLATE css js
ExpiresActive On
ExpiresByType application/x-javascript A2592000

I just finished my weekend project cached-webpgr.js which uses the localStorage / web storage to cache JavaScript files. This approach is very fast. My small test showed

• Loading jQuery from CDN: Chrome 268ms, FireFox: 200ms
• Loading jQuery from localStorage: Chrome 47ms, FireFox 14ms

The code to achieve that is tiny, you can check it out at my Github project https://github.com/webpgr/cached-webpgr.js

Here is a full example how to use it.

The complete library:

function _cacheScript(c,d,e){var a=new XMLHttpRequest;a.onreadystatechange=function(){4==a.readyState&&(200==a.status?localStorage.setItem(c,JSON.stringify({content:a.responseText,version:d})):console.warn("error loading "+e))};a.open("GET",e,!0);a.send()}function _loadScript(c,d,e,a){var b=document.createElement("script");b.readyState?b.onreadystatechange=function(){if("loaded"==b.readyState||"complete"==b.readyState)b.onreadystatechange=null,_cacheScript(d,e,c),a&&a()}:b.onload=function(){_cacheScript(d,e,c);a&&a()};b.setAttribute("src",c);document.getElementsByTagName("head")[0].appendChild(b)}function _injectScript(c,d,e,a){var b=document.createElement("script");b.type="text/javascript";c=JSON.parse(c);var f=document.createTextNode(c.content);b.appendChild(f);document.getElementsByTagName("head")[0].appendChild(b);c.version!=e&&localStorage.removeItem(d);a&&a()}function requireScript(c,d,e,a){var b=localStorage.getItem(c);null==b?_loadScript(e,c,d,a):_injectScript(b,c,d,a)};

Calling the library

requireScript('jquery', '1.11.2', 'http://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js', function(){
    requireScript('examplejs', '0.0.3', 'example.js');
});

From PHP:

function OutputJs($Content) 
{   
    ob_start();
    echo $Content;
    $expires = DAY_IN_S; // 60 * 60 * 24 ... defined elsewhere
    header("Content-type: x-javascript");
    header('Content-Length: ' . ob_get_length());
    header('Cache-Control: max-age='.$expires.', must-revalidate');
    header('Pragma: public');
    header('Expires: '. gmdate('D, d M Y H:i:s', time()+$expires).'GMT');
    ob_end_flush();
    return; 
}   

works for me.

As a developer you'll probably quickly run into the situation that you don't want files cached, in which case see Help with aggressive JavaScript caching

In your Apache .htaccess file:

#Create filter to match files you want to cache 
<Files *.js>
Header add "Cache-Control" "max-age=604800"
</Files>

I wrote about it here also:

http://betterexplained.com/articles/how-to-optimize-your-site-with-http-caching/

I am heavily tempted to close this as a duplicate; this question appears to be answered in many different ways all over the site:

• https://stackoverflow.com/questions/301284/will-a-script-in-htmls-script-tag-with-extension-php-be-cached
• https://stackoverflow.com/questions/206783/when-does-browser-automatically-clear-javascript-cache
• https://stackoverflow.com/questions/54475/help-with-aggressive-javascript-caching
• https://stackoverflow.com/questions/49547/making-sure-a-web-page-is-not-cached-across-all-browsers
• https://stackoverflow.com/questions/3224/how-can-i-make-the-browser-see-css-and-javascript-changes

The best (and only) method is to set correct HTTP headers, specifically these ones: "Expires", "Last-Modified", and "Cache-Control". How to do it depends on the server software you use.

In http://lazutkin.com/blog/2007/feb/1/improving-performance/">Improving performance… look for "Optimization on server side" for general considerations and relevant links and for "Client-side cache" for the Apache-specific advice.

If you are a fan of http://sysoev.ru/nginx/">nginx</a> (or http://nginx.net/">nginx in plain English) like I am, you can easily configure it too:

location /images {
  ...
  expires 4h;
}

In the example above any file from /images/ will be cached on the client for 4 hours.

Now when you know right words to look for (HTTP headers "Expires", "Last-Modified", and "Cache-Control"), just peruse the documentation of the web server you use.

I have a simple system that is pure JavaScript. It checks for changes in a simple text file that is never cached. When you upload a new version this file is changed. Just put the following JS at the top of the page.

        (function(url, storageName) {
            var fromStorage = localStorage.getItem(storageName);
            var fullUrl = url + "?rand=" + (Math.floor(Math.random() * 100000000));
            getUrl(function(fromUrl) {
//                   first load
                if (!fromStorage) {
                    localStorage.setItem(storageName, fromUrl);
                    return;
                }
//                    old file
                if (fromStorage === fromUrl) {
                    return;
                }
                // files updated
                localStorage.setItem(storageName, fromUrl);
                location.reload(true);
            });
            function getUrl(fn) {
                var xmlhttp = new XMLHttpRequest();
                xmlhttp.open("GET", fullUrl, true);
                xmlhttp.send();
                xmlhttp.onreadystatechange = function() {
                    if (xmlhttp.readyState === XMLHttpRequest.DONE) {
                        if (xmlhttp.status === 200 || xmlhttp.status === 2) {
                            fn(xmlhttp.responseText);
                        }
                        else if (xmlhttp.status === 400) {
                            throw 'unable to load file for cache check ' +  url;
                        }
                        else {
                           throw 'unable to load file for cache check ' +  url;
                        }
                    }
                };
            }
            ;
        })("version.txt", "version");

just replace the "version.txt" with your file that is always run and "version" with the name you want to use for your local storage.