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JavaScript中全局變量、函數內變量以及常量表達式的效率測試

來源：程序員人生發布時間：2014-06-17 08:01:18 閱讀次數：3418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：可修改后代碼再運行!

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

這么看的話使用函數內的本地變量時效率要比用全局變量或者常量表達式高出許多。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，但創建正則表達式就比起用全局變量要慢上很多了。

觀察數字還可以發現：在Google Chrome、Opera和IE中，用屬性和利用全局變量相差不大（Opera的表現也許是其垃圾回收機制造成的），從這個現象上看的話，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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