how to trigger a function when a document starts loading in jquery code example

Example 1: jquery onload event

// A $( document ).ready() block.
$( document ).ready(function() {
    console.log( "ready!" );
});

Example 2: jquery check if document loaded

jQuery offers several ways to attach a function that will run when the DOM is ready. All of the following syntaxes are equivalent:

$( handler )
$( document ).ready( handler )
$( "document" ).ready( handler )
$( "img" ).ready( handler )
$().ready( handler )