Detect if script has already loaded or not

Question

It seems that helloworld.js gets loaded multiple times based on the number of times I click #load. I say this because when I look at Google Chromes Developer Tools Network tab, it shows helloworld.js as many times as I click #load.

$(document).ready(function() {

    $("#load").click(function(){
        $.getScript('helloworld.js', function() {
            hello();
        });
    });

});

The hello() function looks like this:

function hello(){
    alert("hello");
}

Is it possible to detect if helloworld.js has already loaded?

So if it hasn't loaded, load it, and if it has loaded, don't load it.

This is what Developer Tools currently shows me if I click the #load button 4 times:

Solution 1

Another option is letting .getScript() run but let it take the script from browser's cache so you won't have it reloaded each and every time.

To achieve this, add such code:

$.ajaxSetup({
    cache: true
});

This is taken from the documentation page.

Solution 2

Set a flag when file loaded successfully. If flag is set then skip the file loading again.

Try this code,

    var isLoaded = 0; //Set the flag OFF 

    $(document).ready(function() {

        $("#load").click(function(){
            if(isLoaded){ //If flag is ON then return false
                alert("File already loaded");
                return false;
            }
            $.getScript('helloworld.js', function() {
                isLoaded = 1; //Turn ON the flag
                hello();

            });
        });

    });

Solution 3

So why not only fire the event once like this:

$("#load").one("click", function() {
   $load = $(this);
   $.getScript('helloworld.js', function() {
       hello();
       // bind hello to the click event of load for subsequent calls
       $load.on('click', hello); 
   });
});

That would prevent subsequent loads and avoids the use of a global

Solution 4

You could create a helper function:

var getScript = (function() {
  var loadedFiles = {};
  return function(filename, callback) {
    if(loadedFiles[filename]) {
      callback();
    } else {
      $.getScript(filename, function() {
        loadedFiles[filename] = true;
        callback();
      });
    }
  };
})();

View more solutions

Comments

[Kevin B]

And to add, use $("#load").click(hello) after hello() to make subsequent clicks to #load call the hello method without re-loading the js.

[Šime Vidas]

Implementing such a cache system is not that trivial. For instance, what if getScript is invoked multiple times for the same file name before the file in question is retrieved and executed?

[pimvdb]

@Šime Vidas: Good point. On the other hand, that would only happen if you click multiple times very fast after each other, which may not be worth caring about.

[oshirowanen]

@Gabe, I need to run the hello() function many times, but need to load the helloworld.js only once.

[Šime Vidas]

As both my parents regularly double-click on buttons, and links on web-pages, I think this scenario would occur more often than you think :P

[Rajshekar Reddy]

If the script file has some event binding code wouldn't the event bind multiple times, each time the file is loaded or taken from the cache?

[Shadow The Kid Wizard]

@Reddy that is true, didn't think of it. My answer is an alternative to this other answer, which advice to use the .one() which I must admit is better in this case.

[Rajshekar Reddy]

no problem in your answer at all. Was just curious to know, Because I have a issue where I load a html page which has local scripts within it and the events are binding multiple times. Just wanted to know if it is the same when read from cache.

[Shadow The Kid Wizard]

@Reddy yeah, it will probably bind them again indeed, though it's worth checking in depth.