CODEWITHSUNDEEP
javascriptasynchronous
joshhunt
joshhunt

Reputation: 5335

Javascript: Run function when page has finished loading, script is loading async

I can't find a reliable way to find when the page has finished loading. My current code looks like this:

function pageLoaded(callback) {
    function completed() {
        document.removeEventListener('DOMContentLoaded', completed);
        callback();
    }

    if (document.readyState === 'complete') {
        callback();
    } else {
        document.addEventListener('DOMContentLoaded', completed);
    }
}

The issue is that DOMContentLoaded gets fired before document.readyState gets set to 'complete' which means that the callback never gets called.

When my function runs DOMContentLoaded has already been fired but document.readyState === 'interactive'. However I don't think I can run my callback when document.readyState === 'interactive' due to these possible issues.

Unfortunately I don't think I can make a demo due to the use of async. Also this doesn't happen 100% of the time but it seems to happen always when I do a hard reload (I assume something to do with caching).

Note that I'm loading in my script like this in my <head>:

<script src="script.js" async></script>

Upvotes: 3

Views: 4600

Answers (2)

joshhunt
joshhunt

Reputation: 5335

I couldn't find much useful information on the subject so I decided to do some tests. I set up a Node server that would:

  1. Send the start of an html document
  2. Wait 5 seconds
  3. Send the rest of the html which includes an image

I then recorded the status of document.ready and DOMContentLoaded at each stage. My code:

var http = require('http');

var server = http.createServer(function(req, res) {
    // Send the first part of the html
    res.writeHead(200, {'Content-Type': 'text/html'});
    res.write(
        '<!doctype html>' +
            '<html lang="en">' +
                '<head>' +
                    '<meta charset="utf-8">' +
                    '<meta http-equiv="x-ua-compatible" content="ie=edge">' +
                    '<title>JS Ready Test</title>' +
                    '<meta name="description" content="">' +
                    '<meta name="viewport" content="width=device-width, initial-scale=1">' +

                    '<script>' +
                        'console.log(document.readyState);' +

                        'document.onreadystatechange = function () {' +
                            'console.log(document.readyState);' +
                        '};' +

                        'document.addEventListener("DOMContentLoaded", function() {' +
                            'console.log("DOMContentLoaded");' +
                        '});' +
                    '</script>' +
                '</head>' +
                '<body>');
    // Send a bunch of blank spaces so that the browser will load the buffer, if the buffer is too small the browser will wait for more data
    var str = 'Start';
    for (var i = 0; i < 2000; i++){
      str += ' ';
    }
    res.write(str);

    // Wait 5 seconds and send the rest of the data
    setTimeout(function () {
        res.write('Finish<img src="https://upload.wikimedia.org/wikipedia/commons/3/3d/LARGE_elevation.jpg"></body></html>');
        res.end();
    }, 5000);
});

// Listen on port 3000
server.listen(3000);

Results from my tests

First Buffer
Chrome (v43) / FF (v39) / IE11: document.ready === 'loading'
IE9 / IE10: document.ready === 'interactive'

Final buffer
Chrome / FF / IE11: document.ready === 'interactive', DOMContentLoaded called
IE9 / IE10: No change in document.ready, DOMContentLoaded called

Sub-resources finish loading (in this case the image)
Chrome / FF / IE11: document.ready === 'complete'
IE9 / IE10: document.ready === 'complete'

As you can see IE9 & IE10 set document.ready === 'interactive' too early.

Some possible solutions

1. Ignore IE9 / IE10

if (document.readyState === 'interactive' || document.readyState === 'complete') {
    callback();
} else {
    document.addEventListener('DOMContentLoaded', callback);
}

2. Add the DOMContentLoaded in the <head> of your document outside of your async script. This ensures that it will be attached before it is called.

// In <head>
<script>
    var pageLoaded = false;

    document.addEventListener('DOMContentLoaded', function() {
        pageLoaded = true;
    });
</script>

// In script.js
if (pageLoaded) {
    callback();
} else  {
    document.addEventListener('DOMContentLoaded', callback);
}

3. Fallback to the load event on `window.

if (document.readyState === 'complete') {
    callback();
} else {
    // You would need to add a safety so that your functions don't get called twice
    document.addEventListener('DOMContentLoaded', callback);
    window.addEventListener( "load", callback);
}

Upvotes: 6

Haniel Bitton
Haniel Bitton

Reputation: 84

function pageLoad(callback) {
    if ("function" == typeof callback) {
        if (document.addEventListener) { // Event that fires when the initial HTML document has been completely loaded and parsed
            document.addEventListener("DOMContentLoaded", callback, false);
        } else if (window.attachEvent) { // For IE 8 and below
            window.attachEvent("onload", callback);
        } else if ("function" == typeof window.onload) { // Event that fires when the page has fully loaded including images / scripts etc
            var o = window.onload;
            window.onload = function() {
                o();
                callback();
            };
        } else {
            window.onload = callback;
        }
    }
}

Upvotes: 0

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