jQuery.readyReturns: Thenable
Description: A Promise-like object (or "thenable") that resolves when the document is ready.
version added: 1.8jQuery.ready
As of jQuery 3.0, use of this object is supported via jQuery.when or the native Promise.resolve(). Code should not make assumptions about whether this object is a jQuery.Deferred, native Promise, or some other type of promise object.
See also ready(), which makes use of this-
Examples:
Listen for document ready using jQuery.when.
$.when( $.ready ).then(function() {
// Document is ready.
}); Typical usage involving another promise, using jQuery.when.
$.when(
$.getJSON( "ajax/test.json" ),
$.ready
).done(function( data ) {
// Document is ready.
// Value of test.json is passed as `data`.
}); 
