https://www.w3schools.com/jquery/event_ready.asp
Example
Use ready() to make a function available after the document is loaded:
$(document).ready(function(){
$("button").click(function(){
$("p").slideToggle();
});
});
Definition and Usage
The ready event occurs when the DOM (document object model) has been loaded.
Because this event occurs after the document is ready, it is a good place to have all other jQuery events and functions. Like in the example above.
The ready() method specifies what happens when a ready event occurs.
Tip: The ready() method should not be used together with <body onload="">.
只能在document上使用