我有一个使用$（document）.ready的脚本，但它不使用jQuery中的任何其他内容。我想通过删除jQuery依赖项来减轻它。
如何在不使用jQuery的情况下实现我自己的$（document）.ready功能？我知道，使用window.onload将不同，因为window.onlead在加载所有图像、帧等后启动。
当前回答
2022版本
2022年，您需要做的就是将defer属性放在脚本上，并将其加载到头部！
参考：https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#attr-推迟
<!doctype html>
<html>
<head>
<script src="/script.js" defer></script>
</head>
<body>
<p>In 2022, all you need to do is put the defer attribute on your script, and load it in the head!</p>
</body>
</html>
其他回答
最小且100%工作
我已经从PlainJS中选择了答案，它对我来说很好。它扩展了DOMContentLoaded，因此它可以在所有浏览器中被接受。
此函数等效于jQuery的$（document）.ready（）方法：
document.addEventListener('DOMContentLoaded', function(){
// do something
});
然而，与jQuery不同的是，这段代码只能在现代浏览器中正常运行（IE>8），如果在插入此脚本时（例如通过Ajax）已经呈现了文档，则无法正常运行。因此，我们需要稍微扩展一下：
function run() {
// do something
}
// in case the document is already rendered
if (document.readyState!='loading') run();
// modern browsers
else if (document.addEventListener)
document.addEventListener('DOMContentLoaded', run);
// IE <= 8
else document.attachEvent('onreadystatechange', function(){
if (document.readyState=='complete') run();
});
这基本上涵盖了所有的可能性，是jQuery助手的可行替代品。
适用于所有已知的浏览器（通过BrowserStack测试）。IE6+、Safari 1+、Chrome 1+、Opera等。使用DOMContentLoaded，带有document.dococumentElement.doScroll（）和window.onload的回退。
/*! https://github.com/Kithraya/DOMContentLoaded v1.2.6 | MIT License */
DOMContentLoaded.version = "1.2.6";
function DOMContentLoaded() { "use strict";
var ael = 'addEventListener', rel = 'removeEventListener', aev = 'attachEvent', dev = 'detachEvent';
var alreadyRun = false, // for use in the idempotent function ready()
funcs = arguments;
// old versions of JS return '[object Object]' for null.
function type(obj) { return (obj === null) ? 'null' : Object.prototype.toString.call(obj).slice(8,-1).toLowerCase() }
function microtime() { return + new Date() }
/* document.readyState === 'complete' reports correctly in every browser I have tested, including IE.
But IE6 to 10 don't return the correct readyState values as per the spec:
readyState is sometimes 'interactive', even when the DOM isn't accessible in IE6/7 so checking for the onreadystatechange event like jQuery does is not optimal
readyState is complete at basically the same time as 'window.onload' (they're functionally equivalent, within a few tenths of a second)
Accessing undefined properties of a defined object (document) will not throw an error (in case readyState is undefined).
*/
// Check for IE < 11 via conditional compilation
/// values: 5?: IE5, 5.5?: IE5.5, 5.6/5.7: IE6/7, 5.8: IE8, 9: IE9, 10: IE10, 11*: (IE11 older doc mode), undefined: IE11 / NOT IE
var jscript_version = Number( new Function("/*@cc_on return @_jscript_version; @*\/")() ) || NaN;
// check if the DOM has already loaded
if (document.readyState === 'complete') { ready(null); return; } // here we send null as the readyTime, since we don't know when the DOM became ready.
if (jscript_version < 9) { doIEScrollCheck(); return; } // For IE<9 poll document.documentElement.doScroll(), no further actions are needed.
/*
Chrome, Edge, Firefox, IE9+, Opera 9+, Safari 3.1+, Android Webview, Chrome for Android, Edge Mobile,
Firefox for Android 4+, Opera for Android, iOS Safari, Samsung Internet, etc, support addEventListener
And IE9+ supports 'DOMContentLoaded'
*/
if (document[ael]) {
document[ael]("DOMContentLoaded", ready, false);
window[ael]("load", ready, false); // fallback to the load event in case addEventListener is supported, but not DOMContentLoaded
} else
if (aev in window) { window[aev]('onload', ready);
/* Old Opera has a default of window.attachEvent being falsy, so we use the in operator instead
https://dev.opera.com/blog/window-event-attachevent-detachevent-script-onreadystatechange/
Honestly if somebody is using a browser so outdated AND obscure (like Opera 7 where neither addEventListener
nor "DOMContLoaded" is supported, they deserve to wait for the full page).
I CBA testing whether readyState === 'interactive' is truly interactive in browsers designed in 2003. I just assume it isn't (like in IE6-8).
*/
} else { // fallback to queue window.onload that will always work
addOnload(ready);
}
// This function allows us to preserve any original window.onload handlers (in super old browsers where this is even necessary),
// while keeping the option to chain onloads, and dequeue them.
function addOnload(fn) { var prev = window.onload; // old window.onload, which could be set by this function, or elsewhere
// we add a function queue list to allow for dequeueing
// addOnload.queue is the queue of functions that we will run when when the DOM is ready
if ( type( addOnload.queue ) !== 'array') { addOnload.queue = [];
if ( type(prev) === 'function') { addOnload.queue.push( prev ); } // add the previously defined event handler
}
if (typeof fn === 'function') { addOnload.queue.push(fn) }
window.onload = function() { // iterate through the queued functions
for (var i = 0; i < addOnload.queue.length; i++) { addOnload.queue[i]() }
};
}
// remove a queued window.onload function from the chain (simplified);
function dequeueOnload(fn) { var q = addOnload.queue, i = 0;
// sort through the queued functions in addOnload.queue until we find `fn`
if (type( q ) === 'array') { // if found, remove from the queue
for (; i < q.length; i++) { ;;(fn === q[i]) ? q.splice(i, 1) : 0; } // void( (fn === q[i]) ? q.splice(i, 1) : 0 )
}
}
function ready(ev) { // idempotent event handler function
if (alreadyRun) {return} alreadyRun = true;
// this time is when the DOM has loaded (or if all else fails, when it was actually possible to inference the DOM has loaded via a 'load' event)
// perhaps this should be `null` if we have to inference readyTime via a 'load' event, but this functionality is better.
var readyTime = microtime();
detach(); // detach any event handlers
// run the functions
for (var i=0; i < funcs.length; i++) { var func = funcs[i];
if (type(func) === 'function') {
func.call(document, { 'readyTime': (ev === null ? null : readyTime), 'funcExecuteTime': microtime() }, func);
// jquery calls 'ready' with `this` being set to document, so we'll do the same.
}
}
}
function detach() {
if (document[rel]) {
document[rel]("DOMContentLoaded", ready); window[rel]("load", ready);
} else
if (dev in window) { window[dev]("onload", ready); }
else {
dequeueOnload(ready);
}
}
function doIEScrollCheck() { // for use in IE < 9 only.
if ( window.frameElement ) {
// we're in an <iframe> or similar
// the document.documentElemeent.doScroll technique does not work if we're not at the top-level (parent document)
try { window.attachEvent("onload", ready); } catch (e) { } // attach to onload if were in an <iframe> in IE as there's no way to tell otherwise
return;
}
try {
document.documentElement.doScroll('left'); // when this statement no longer throws, the DOM is accessible in old IE
} catch(error) {
setTimeout(function() {
(document.readyState === 'complete') ? ready() : doIEScrollCheck();
}, 50);
return;
}
ready();
}
}
用法：
<script>
DOMContentLoaded(function(e) { console.log(e) });
</script>
实际上，如果您只关心Internet Explorer 9+，那么这段代码就足以取代jQuery.ready：
document.addEventListener("DOMContentLoaded", callback);
如果您担心Internet Explorer 6和一些非常奇怪和罕见的浏览器，这将奏效：
domReady: function (callback) {
// Mozilla, Opera and WebKit
if (document.addEventListener) {
document.addEventListener("DOMContentLoaded", callback, false);
// If Internet Explorer, the event model is used
} else if (document.attachEvent) {
document.attachEvent("onreadystatechange", function() {
if (document.readyState === "complete" ) {
callback();
}
});
// A fallback to window.onload, that will always work
} else {
var oldOnload = window.onload;
window.onload = function () {
oldOnload && oldOnload();
callback();
}
}
},
这个解决方案怎么样？
// other onload attached earlier
window.onload=function() {
alert('test');
};
tmpPreviousFunction=window.onload ? window.onload : null;
// our onload function
window.onload=function() {
alert('another message');
// execute previous one
if (tmpPreviousFunction) tmpPreviousFunction();
};
穷人的解决方案：
var checkLoad = function() {
document.readyState !== "complete" ? setTimeout(checkLoad, 11) : alert("loaded!");
};
checkLoad();
查看Fiddle
添加了这个，我想更好一点，自己的范围，非递归
(function(){
var tId = setInterval(function() {
if (document.readyState == "complete") onComplete()
}, 11);
function onComplete(){
clearInterval(tId);
alert("loaded!");
};
})()
查看Fiddle
