JavaScript性能提升学习

<script type="text/javascript" src="http://yui.yahooapis.com/combo?2.7.0/build/yahoo/yahoo-min.js&2.7.0/build/event/event-min.js "></script>

<script defer>
    alert("defer");
</script>
<script>
    alert("script");
</script>
<script>
    window.onload = function(){
        alert("load");
    };
</script>

var xhr = new XMLHttpRequest();
xhr.open("get", "file1.js", true);
xhr.onreadystatechange = function(){
    if (xhr.readyState == 4){
        if (xhr.status >= 200 && xhr.status < 300 || xhr.status == 304){
            var script = document.createElement("script");
            script.type = "text/javascript";
            script.text = xhr.responseText;
            document.body.appendChild(script);
        }
    }
};
xhr.send(null);

function loadScript(url, callback){
    var script = document.createElement("script")
    script.type = "text/javascript";
    if (script.readyState){ //IE
        script.onreadystatechange = function(){
            if (script.readyState == "loaded" || script.readyState == "complete"){
                script.onreadystatechange = null;
                callback();
            }
        };
    } else { //Others
        script.onload = function(){
            callback();
        };
    }
    script.src = url;
    document.getElementsByTagName("head")[0].appendChild(script);
}
loadScript("the-rest.js", function(){
    Application.init();
});

var doc = document;
doc.X……
doc.XX……
doc.XXX……

var alldivs = document.getElementsByTagName('div');
for (var i = 0; i < alldivs.length; i++) {
    document.body.appendChild(document.createElement('div'))
}

属性名                     被替代的属性
children                        childNodes
childElementCount           childNodes.length
firstElementChild           firstChild
lastElementChild            lastChild
nextElementSibling          nextSibling
previousElementSibling      previousSibling

document.querySelector('#menu a');

try{
    // 递归程序
}catch(e){}

String.prototype.trim = function(){
    var str = this.replace(/^\s+/,""),
        end = str.length - 1,
        ws = /\s/;
    while(ws.test(str.charAt(end))){
        end--;
    }
    return str.slice(0,end+1);
}

function timedProcessArray(items,process,callback){
    var todo = items.concat(); // 克隆原始数组
    setTimeout(function(){
        var start = +new Date();
        do{
            process(todo.shift());
        }while(todo.length > 0 && (+new Date()-start<50));
        if(todo.length>0){
            setTimeout(arguments.callee,25);
        }else{
            callback(items);
        }
    },25);
}

// 实时处理请求响应数据
var req = new XMLHttpRequest();
var getLatestPacketInterval, lastLength = 0;
req.open('GET', 'rollup_images.php', true);
req.onreadystatechange = readyStateHandler;
req.send(null);
function readyStateHandler{
    // 传输中
    if (req.readyState === 3 && getLatestPacketInterval === null) {
        // Start polling.
        getLatestPacketInterval = window.setInterval(function() {
            getLatestPacket();
        }, 15);
    }
    // 传输结束
    if (req.readyState === 4) {
        // Stop polling.
        clearInterval(getLatestPacketInterval);
        // Get the last packet.
        getLatestPacket();
    }
}
function getLatestPacket() {
    var length = req.responseText.length;
    var packet = req.responseText.substring(lastLength, length);
    processPacket(packet);
    lastLength = length;
}

req.onerror = function() {
    setTimeout(function() {
        xhrPost(url, params, callback);
    }, 1000);
};

var localCache = {};
function xhrRequest(url, callback) {
    // Check the local cache for this URL.
    if (localCache[url]) {
        callback.success(localCache[url]);
        return;
    }
    // If this URL wasn't found in the cache, make the request.
    var req = createXhrObject();
    req.onerror = function() {
        callback.error();
    };
    req.onreadystatechange = function() {
        if (req.readyState == 4) {
            if (req.responseText === '' || req.status == '404') {
                callback.error();
                return;
            }
            // Store the response on the local cache.
            localCache[url] = req.responseText;
            callback.success(req.responseText);
        }
    };
    req.open("GET", url, true);
    req.send(null);
}

delete localCache['/user/friendlist/'];
delete localCache['/user/contactlist/'];