Why Ios Mobile Cannot Use Focus()

I'm trying to use the javascript function focus(), but it does not work on safari mobile. How do I fix this problem?

$( document ).ready( function() {

    setTimeout( $( '#username' ).focus(), 0 );

});

Now, I don't exactly know how it works; maybe some rendering issue. But it works for sure.

If you want VanillaJS:

window.onload = function() {

    setTimeout( document
                  .getElementById( 'username' ).focus(), 0 );

}

I don't know if the VanillaJS version will work.