概念:在javascript裏函數可以動態定義,如果創建了一個新函數並且將其分配給保存了另外函數的同一個變量,那麼新函數覆蓋舊函數。常用於初始化功能。
原理:同名函數覆蓋重寫
適用場景:在固定的應用環境不會發生改變,頻繁使用同一判斷邏輯,一次判定,多次直接使用
來個經典例子:創建XHR請求,需要進行能力檢測,如下:
function createXHR(){
if (typeof XMLHttpRequest != "undefined"){
return new XMLHttpRequest();
} else if (typeof ActiveXObject != "undefined"){
if (typeof arguments.callee.activeXString != "string"){
var versions = ["MSXML2.XMLHttp.6.0", "MSXML2.XMLHttp.3.0",
"MSXML2.XMLHttp"];
for (var i=0,len=versions.length; i < len; i++){
try {
var xhr = new ActiveXObject(versions[i]);
arguments.callee.activeXString = versions[i];
return xhr;
} catch (ex){
//skip
}
}
}
return new ActiveXObject(arguments.callee.activeXString);
} else {
throw new Error("No XHR object available.");
}
}
頁面有多次請求,每次創建XHR都去能力檢測,是不是有些浪費呢,如果我們在第一次調用的時候,重寫函數,剩下的調用不再進行能力檢測,是不是提高代碼性能呢,例子如下
function createXHR() {
if (typeof XMLHttpRequest != "undefined") {
createXHR = function () {
return new XMLHttpRequest();
}
return new XMLHttpRequest();
} else if (typeof ActiveXObject != "undefined") {
var curxhr;
var versions = ["MSXML2.XMLHttp.6.0", "MSXML2.XMLHttp.3.0",
"MSXML2.XMLHttp"];
for (var i = 0, len = versions.length; i < len; i++) {
try {
var xhr = new ActiveXObject(versions[i]);
curxhr = versions[i];
createXHR = function () {
return new ActiveXObject(curxhr);
}
return xhr;
} catch (ex) {
//skip
}
}
} else {
throw new Error("No XHR object available.");
}
}
console.log(createXHR) /* 結果爲原函數
ƒ createXHR() {
if (typeof XMLHttpRequest != "undefined") {
createXHR = function () {
return new XMLHttpRequest();
}
return new XMLHttpRequest();
} else if (typeof ActiveXObject != "undefined") {
var curxhr;
var versions = ["MSXML2.XMLHttp.6.0", "MSXML2.XMLHttp.3.0",
"MSXML2.XMLHttp"];
for (var i = 0, len = versions.length; i < len; i++) {
try {
var xhr = new ActiveXObject(versions[i]);
curxhr = versions[i];
createXHR = function () {
return new ActiveXObject(curxhr);
}
return xhr;
} catch (ex) {
//skip
}
}
} else {
throw new Error("No XHR object available.");
}
}
*/
打印結果爲原函數
createXHR(); //調用
console.log(createXHR) //結果爲執行了一次後重寫的得到的函數 ƒ () { return new XMLHttpRequest(); }