How to know if window “load” event was fired already

I'm writing a Javascript script. This script will probably be loaded asynchronously (AMD format).

In this script, I'd like to do nothing important until the window.load event was fired. So I listen to the window "load" event.

But if the script is loaded after window.load event... how can I know window.load was already fired?

And of course I don't want to add something in any other scripts (they are all loaded async, the problem is the same) :)

Edit :

Imagine an HTML doc with no Javascript in it at all.

Than someone insert in this doc a tag, and this script tag loads my Javascript file.

This will execute my script.

How this script can know if window.load was already fired ?

No jQuery, not any script in the HTML doc before mine.

Is it possible to know ??

I found the window.document.readystate property. This property is for document "ready" event I guess, not for window "load". Is there anything similar for window "load" event ?

Answers:

Answer

Browser navigation performance loadEventEnd metric can be used to determinate if load event was triggered:

let navData = window.performance.getEntriesByType("navigation");
if (navData.length > 0 && navData[0].loadEventEnd > 0)
{
    console.log('Document is loaded');
} else {
    console.log('Document is not loaded');
}
Answer

The easiest solution might be checking for document.readyState == 'complete', see http://www.w3schools.com/jsref/prop_doc_readystate.asp

Answer

Quick Answer

To quickly answer the question's title:

document.readyState === 'complete'

Deeper Example

Below is a nice helper if you want to call code upon a window load, while still handling the case where the window may have already loaded by the time your code runs.

function winLoad(callback) {
  if (document.readyState === 'complete') {
    callback();
  } else {
    window.addEventListener("load", callback);
  }
}

winLoad(function() {
  console.log('Window is loaded');
});

Note: code snippets on here actually don't run in the same window context so document.readyState === 'complete' actually evaluates to false when you run this. If you put the same into your console right now for this window it should evaluate as true.

See also: What is the non-jQuery equivalent of '$(document).ready()'?

Answer

Here is my answer:

fiddle

window.addEventListener("load", function () {
    window.loaded = true;
});

function logLoaded() {
    console.log("loaded");
}

(function listen () {
    if (window.loaded) {
        logLoaded();
    } else {
        console.log("notLoaded");
        window.setTimeout(listen, 50);
    }
})();

You can read about addEventListener() and its compatibility (it is new to the ECMAScript 5 spec) here. It is the new "preferred" way to do things going forward.

You can read about Immediately Invoked Function Expressions (IIFE) (alternately, self-invoked anonymous functions or immediately invoked anonymous functions) here.

EDIT: Here is a good answer already on StackOverflow:

How to check if DOM is ready without a framework?

If you specifically want to know if the DOM load event has fired, set a global variable in a DOM 'load' event handler and then check for its existence when your new code loads.

// in 'load' event handler
window.domLoadEventFired = true;

// in code you are loading asynchronously
if (typeof window.domLoadEventFired !== undefined) {
    // ...
}
Answer

If you don't want to use jQuery, the logic it uses is:

if( !document.body )
    setTimeout( checkAgain, 1 );

So between the windows loaded event and checking if the body property of the document is available, you can check if the DOM is ready

Answer

Based on @CTS_AE's approach, I have put together a solution for envrionments where:

  • window.addEventListener('load', activateMyFunction); and
  • window.addEventListener('DOMContentLoaded', activateMyFunction);

don't work.

It requires a single character substitution (eg. from

window.addEventListener('load', activateMyFunction);

to

window_addEventListener('load', activateMyFunction);)

The function window_addEventListener() looks like this:

const window_addEventListener = (eventName, callback, useCapture = false) => {

  if ((document.readyState === 'interactive') || (document.readyState === 'complete'))   {

    callback();
  }
}
Answer

what about overriding window.load?

window._load = window.load;
window.load = function(){
  window.loaded = true;
  window._load();
}

Then check for

if (window.loaded != undefined && window.loaded == true){
    //do stuff
}

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