值得在Rock Solid addEvent（）和http://www.braksator.com/how-to-make-your-own-jquery.

这是网站崩溃时的代码

function addEvent(obj, type, fn) {
    if (obj.addEventListener) {
        obj.addEventListener(type, fn, false);
        EventCache.add(obj, type, fn);
    }
    else if (obj.attachEvent) {
        obj["e"+type+fn] = fn;
        obj[type+fn] = function() { obj["e"+type+fn]( window.event ); }
        obj.attachEvent( "on"+type, obj[type+fn] );
        EventCache.add(obj, type, fn);
    }
    else {
        obj["on"+type] = obj["e"+type+fn];
    }
}

var EventCache = function(){
    var listEvents = [];
    return {
        listEvents : listEvents,
        add : function(node, sEventName, fHandler){
            listEvents.push(arguments);
        },
        flush : function(){
            var i, item;
            for(i = listEvents.length - 1; i >= 0; i = i - 1){
                item = listEvents[i];
                if(item[0].removeEventListener){
                    item[0].removeEventListener(item[1], item[2], item[3]);
                };
                if(item[1].substring(0, 2) != "on"){
                    item[1] = "on" + item[1];
                };
                if(item[0].detachEvent){
                    item[0].detachEvent(item[1], item[2]);
                };
                item[0][item[1]] = null;
            };
        }
    };
}();

// Usage
addEvent(window, 'unload', EventCache.flush);
addEvent(window, 'load', function(){alert("I'm ready");});

(function(f){
  if(document.readyState != "loading") f();
  else document.addEventListener("DOMContentLoaded", f);
})(function(){
  console.log("The Document is ready");
});

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>

试试看：

function ready(callback){
    if(typeof callback === "function"){
        document.addEventListener("DOMContentLoaded", callback);
        window.addEventListener("load", callback);
    }else{
        throw new Error("Sorry, I can not run this!");
    }
}
ready(function(){
    console.log("It worked!");
});

只需将其添加到HTML页面的底部。。。

<script>
    Your_Function();
</script>

因为，HTML文档是按上下顺序解析的。

使用纯JavaScript的最简单方法。没有jQuery：

document.addEventListener("DOMContentLoaded", function(event) {
   // Your code to run since DOM is loaded and ready
});