有javascript怎么把一个数字，自动转换成有千分位的形式？？？

比如：1123456.78 转换为 1,123,456.78
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 1<script language="javascript">   
 2function commafy(form){   
 3var re=/(-?\d+)(\d{3})/   
 4var num=form.entry.value   
 5while(re.test(num)){   
 6num=num.replace(re,"$1,$2")   
 7}   
 8form.entry.value=num   
 9}   
10</script>

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 1<body>
 2<script language="javascript">   
 3function commafy(num){   
 4num = num+"";   
 5var re=/(-?\d+)(\d{3})/   
 6while(re.test(num)){   
 7num=num.replace(re,"$1,$2")   
 8}   
 9return num;   
10}   
11</script>
12<input onclick="alert(commafy(123456789.01))" type="button" value="commafy"/>
13</body>