学步园

• setTimeout (slow, takes about 10 sec)
• img.onerror (data:uri)
• script.onreadystatechange
• script.onload (data:uri)
• xhr.onreadystatechange (data:text/plain,foo)
• self.postMessage

javascript中实现这个功能的是来自BOM的一个函数setTimeout，但相关的DOM操作也提供了一系列实现。如XMLHttpRequest对象与script标签的onreadystatechange回调，image的onload与onerror回调，iframe的onload，DOM元素的事件回调，HTML5的跨域消息传送postMessage，QuickTime与flash对象的加载……

setTimeout的零秒延迟在前些年时间被国内宣扬得特别厉害，但setTimeout是所有延迟中最慢的，最少要花上10多毫秒，如果用setTimeout来开发特效，这特效会运行得比较慢。下面是一个性能测试：

 
function async(callback) { 
setTimeout(callback, 0); 
} 

 
function async(callback) { 
var img = new Image; 
img.addEventListener('error', callback, false); 
img.src = 'data:,foo'; 
} 

 
function async(callback) { 
var script = document.createElement("script"); 
script.type = "text/javascript"; 
script.src = "javascript:"; 
script.onreadystatechange = function () { 
document.body.removeChild(script); 
callback(); 
} 
document.body.appendChild(script); 
} 

 
function async(callback) { 
var script = document.createElement('script'); 
script.onload = function() { 
document.body.removeChild(script); 
callback(); 
} 
script.src = 'data:text/javascript,'; 
document.body.appendChild(script); 
} 

 
function async(callback) { 
var xhr = new XMLHttpRequest; 
xhr.open('GET','data:text/plain,foo',true); 
xhr.onreadystatechange = function() { 
xhr.onreadystatechange = null; 
callback(); 
}; 
xhr.send(null); 
} 

 
function async(callback) { 
var n = ++async.count; 
window.addEventListener('message',function(e){ 
if (e.data == n) { 
window.removeEventListener('message', arguments.callee,false); 
callback(); 
} 
},false); 
window.postMessage(n, location.protocol + "//" + location.host); 
} 
async.count = 0;