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JavaScript中全局變量、函數(shù)內(nèi)變量以及常量表達(dá)式的效率測試

來源：程序員人生發(fā)布時間：2014-06-17 08:01:18 閱讀次數(shù)：3411次

<html><body><script type="text/javascript">var r, s = "this is a very very looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong string", x = /^s+|s+$/g;document.writeln("字符串測試 ");(function(){ document.write("全局->全局："); var t = new Date(); for(var i = 0; i < 3000000; i++){ r = s; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("全局->本地："); var s = window.s; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = s; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("常量->本地："); var r; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = "this is a very very looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong string"; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("屬性->本地："); var r; var s = { 0: window.s }; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = s[0]; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("本地->本地："); var r; var s = window.s; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = s; } document.write(new Date() - t); document.writeln(" ");})();document.writeln("正則表達(dá)式測試 ");(function(){ document.write("全局->全局："); var t = new Date(); for(var i = 0; i < 3000000; i++){ r = x; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("全局->本地："); var x = window.x; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = x; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("常量->本地："); var r; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = /^s+|s+$/g; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("屬性->本地："); var r; var x = { 0: window.x }; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = x[0]; } document.write(new Date() - t); document.writeln(" ");})();(function(){ document.write("本地->本地："); var r; var x = window.x; var t = new Date(); for(var i = 0; i < 3000000; i++){ r = x; } document.write(new Date() - t); document.writeln(" ");})();</script></body></html>
提示：可修改后代碼再運(yùn)行!

注意：在IE中進(jìn)行這個測試時需要先改變IE的"最大腳本步數(shù)"：
[HKEY_CURRENT_USERSoftwareMicrosoftInternet ExplorerStyles]
"MaxScriptStatements"=dword:ffffffff

這么看的話使用函數(shù)內(nèi)的本地變量時效率要比用全局變量或者常量表達(dá)式高出許多。而屬性的效率就糟得很恐怖了，在Firefox和Safari中居然比利用全局變量還慢上好多倍。

直接用字符串常量要比利用全局變量快，但創(chuàng)建正則表達(dá)式就比起用全局變量要慢上很多了。

觀察數(shù)字還可以發(fā)現(xiàn)：在Google Chrome、Opera和IE中，用屬性和利用全局變量相差不大（Opera的表現(xiàn)也許是其垃圾回收機(jī)制造成的），從這個現(xiàn)象上看的話，這三種瀏覽器中所謂的全局變量有可能根本就是全局對象的屬性。

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