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

2019-11-21 00:58:42

字體：大中小

供稿：網(wǎng)友

<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>

[Ctrl+A 全選注:如需引入外部Js需刷新才能執(zhí)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(shù)"：
[HKEY_CURRENT_USER/Software/Microsoft/Internet Explorer/Styles]
"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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