Problems using inline functions in JavaScript_javascript tips
前段时间被IE和JavaScript脚本引擎的Memory Leak问题弄得郁闷坏了，不过幸好现在总算是柳暗花明了，并且找到了一些IE中使用脚本避免ML问题的方法。继续研究JavaScript的编写，有发现一些不算ML问题，但是可以节约IE内存使用的方法，在此和大家讨论讨论。
我们在JavaScript中编写代码，对于定义函数的语句：
function foo(){ // TODO: . . . return x;}
可以说是在熟悉不过了。当然除了这种定义函数的方法，我们还有另外几种方法也能定义函数：
var foo = function(){ // TODO: . . . return x;}
var foo = new Function('{/*todo*/return x;}');
后两种方法定义的JavaScript函数，在调用起来和第一种没有任何效果上的区别。
不过由于JavaScript是解释性语言，当我们定义一个函数的时候，解析引擎生成一个Function对象实例，然后把函数内容保存下来。所以每执行一次函数定义语句，就会生成一个函数。而不像编译语言，一个函数编译一次后就被任何语句调用。啊？难道JavaScript不能调用定义好的函数？不是这个意思了，当我们在制作JavaScript控件时，如果动态输出DHTML来作为控件的内容，就容易出现这样的问题。比如我们在一个HTML对象生成过程中，使用了inline方式定义的函数，那么这个元素生成几次，那个函数也就要同时生成几次。
function TestObject.prototype.Render(doc, id) { var span = doc.createElement('SPAN'); span.Object = this; this.m_Element = span; if ( id == "NamedMethod" ) { span.onclick = asdf; } else { span.onclick = function() { var asdf01 = ['a', 's', 'd', 'f']; var asdf02 = ['a', 's', 'd', 'f']; var asdf03 = ['a', 's', 'd', 'f']; var asdf04 = ['a', 's', 'd', 'f']; var asdf05 = ['a', 's', 'd', 'f']; var asdf06 = ['a', 's', 'd', 'f']; var asdf07 = ['a', 's', 'd', 'f']; var asdf08 = ['a', 's', 'd', 'f']; var asdf09 = ['a', 's', 'd', 'f']; var asdf10 = ['a', 's', 'd', 'f']; var asdf11 = ['a', 's', 'd', 'f']; var asdf12 = ['a', 's', 'd', 'f']; }; } span.Name = this.m_Description; span.innerText = this.m_Name; span.style.display = 'block'; return span; }
函数span.onclick = function()中的内容是用来占位置的，这样inline方式定义函数，每次Render()都就会生成一个新的函数对象。使用inline方式有什么不好呢？当对象实例多了的时候，会很明显的浪费内存空间呀，试验数据如下：
| Normal Method | Inline Method | |
|---|---|---|
| Initialized | 27.4 M | 27.4 M |
| Rendered | 33.4 M | 35.2 M |
// Amount of memory consumed by IE (PM VM)
There is not much difference in absolute memory consumption alone, but if you look at relative memory consumption: (35.2 -33.4)/(33.4-27.4) = 30% !!!, which is still very impressive, and the larger the method itself, the more redundant data will be inline.
附测试代码：
<html><head> <title>JScript Function Spendingtitle> <meta name="author" content="birdshome@博客园" /> head><body onunload="ReleaseElements()"> <button id="NamedMethod" onclick="GenerateObjects(this)"> Append Normal Elementsbutton> <button id="AnonymousMethod" onclick="GenerateObjects(this)"> Append Inline Elementsbutton> <div id="container"> div>
<script language="Javascript">
function GenerateObjects(elmt)
{ var room = document.getElementById('container'); for ( var i=0 ; i < 1000 ; ++i ) { var obj = new TestObject('__Object__' i); room.appendChild(obj.Render(document, elmt.id));
} }function TestObject(name){ this.m_Name = name; this.m_Description = ''; this.m_Element = null; this.toString = function() { return '[class TestObject]'; }}function TestObject.prototype.Render(doc, id){ var span = doc.createElement('SPAN'); span.Object = this; this.m_Element = span; if ( id == "NamedMethod" ) { span.onclick = asdf; } else { span.onclick = function() { var asdf01 = ['a', 's', 'd', 'f']; var asdf02 = ['a', 's', 'd', 'f']; var asdf03 = ['a', 's', 'd', 'f']; var asdf04 = ['a', 's', 'd', 'f']; var asdf05 = ['a', 's', 'd', 'f']; var asdf06 = ['a', 's', 'd', 'f']; var asdf07 = ['a', 's', 'd', 'f']; var asdf08 = ['a', 's', 'd', 'f']; var asdf09 = ['a', 's', 'd', 'f']; var asdf10 = ['a', 's', 'd', 'f']; var asdf11 = ['a', 's', 'd', 'f']; var asdf12 = ['a', 's', 'd', 'f']; }; } span.Name = this.m_Description; span.innerText = this.m_Name; span.style.display = 'block'; return span;}function asdf(){ var asdf01 = ['a', 's', 'd', 'f']; var asdf02 = ['a', 's', 'd', 'f']; var asdf03 = ['a', 's', 'd', 'f']; var asdf04 = ['a', 's', 'd', 'f']; var asdf05 = ['a', 's', 'd', 'f']; var asdf06 = ['a', 's', 'd', 'f']; var asdf07 = ['a', 's', 'd', 'f']; var asdf08 = ['a', 's', 'd', 'f']; var asdf09 = ['a', 's', 'd', 'f']; var asdf10 = ['a', 's', 'd', 'f']; var asdf11 = ['a', 's', 'd', 'f']; var asdf12 = ['a', 's', 'd', 'f'];
}script> <script language="javascript">function ReleaseElements(){ var room = document.getElementById('container'); var spans = room.all.tags('SPAN'); for ( var i=0 ; i < spans.length ; ++i ) { spans[i].Object = ''; }} script>body>html>
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