https://learn.jquery.com/using-jquery-core/document-ready/

A page can't be manipulated safely until the document is "ready." jQuery detects this state of readiness for you. Code included inside $( document ).ready() will only run once the page Document Object Model (DOM) is ready for JavaScript code to execute. Code included inside $( window ).load(function() { ... }) will run once the entire page (images or iframes), not just the DOM, is ready.

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>$( document ).ready()</title>
    <script src="https://code.jquery.com/jquery-1.9.1.min.js"></script>
    <script>
        $(document).ready(function () {
            alert("document loaded");
        });

　　　　//document ready 简写
　　　　$(function() {
    　　　　// ...代码...
　　　　})

$(window).load(function () { alert("window loaded"); }); </script> </head> <body> <iframe src="http://techcrunch.com"></iframe> </body> </html>

Experienced developers sometimes use the shorthand $() for $( document ).ready(). If you are writing code that people who aren't experienced with jQuery may see, it's best to use the long form.

// Shorthand for $( document ).ready()
$(function() {
 console.log( "ready!" );
});

You can also pass a named function to $( document ).ready() instead of passing an anonymous function.

// Passing a named function instead of an anonymous function.

function readyFn( jQuery ) {

// Code to run when the document is ready.

}

$( document ).ready( readyFn );

// or:

$( window ).load( readyFn );

DOMContentLoaded

https://developer.mozilla.org/zh-CN/docs/DOM/DOM_event_reference/DOMContentLoaded

当页面中的文档树解析完成时,在页面的Document对象上,会触发DOMContentLoaded事件.该事件代表了,页面的DOM树已经构建完成,但页面引用的样式表和图像文件,以及包含的框架页面可能还没有加载完成,在页面完全加载完成时,会触发另外一个类似的称为"load"的事件.

该事件会冒泡到当前页面的window对象上.但框架页面中文档加载完成时并不会触发父页面的DOMContentLoaded事件.

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	0.2	1.0 (1.7 or earlier)	9.0	9.0	3.1

<script>
  document.addEventListener("DOMContentLoaded", function(event) {
    console.log("DOM fully loaded and parsed");
  });
</script>

$( document ).ready()&$(window).load()

$( document ).ready()

DOMContentLoaded

浏览器兼容性