为什么 window.onload = launchFullscreen(document.documentElement); 不行？

Question

I use launchFullscreen() function for get page full screen. It's work perfect with button onClick .But It doesn't work with window.onload . Are there are any way to call that function from onload.

window.onload = launchFullscreen(document.documentElement);

function launchFullscreen(element) {
    if(element.requestFullscreen) {
        element.requestFullscreen();
    } else if(element.mozRequestFullScreen) {
        element.mozRequestFullScreen();
    } else if(element.webkitRequestFullscreen) {
        element.webkitRequestFullscreen();
    } else if(element.msRequestFullscreen) {
        element.msRequestFullscreen();
    }
}

Answer 1

See the specification :

If any of the following conditions are true, queue a task to fire an event named fullscreenerror with its bubbles attribute set to true on the context object's node document, and then terminate these steps

…

This algorithm is not allowed to show a pop-up.

Full screen mode may only be triggered at times when it is allowed to show a popup.

You may only show a popup in response to a user event.

A click is a user event.

The document loading is not.

There is no way around this.

---

An an aside, as pointed out in Theo's answer, you are calling launchFullscreen immediately and trying to use its return value (not a function) as the load event handler. In this case, it makes no difference though.

Answer 2

Try this:

window.onload = function() {
launchFullscreen(document.documentElement);
}

Answer 3

I work around the "must be user action" by binding it to a click event on the HTML element.

$('html').click( function() {
  if(!document.fullscreenElement){
    $('html')[0].requestFullscreen();
  }
});

You just need to remember to touch something once it is loaded. Even if you forgot, the first click will put the page to full screen.

ps: Yes, you can easily do this without using jQuery by using proper getElement... functions.