Leading zero 前導零，也就是常在數字前面看到的一串補0，有興趣可以看維基百科的詳細解釋。
直觀的對比就如下：
| 一般數字 | 經過 Leading Zero 處理 |
|---|---|
| 123 | 000000123 |
| 44444 | 000044444 |
| 5566666 | 005566666 |
| 7890 | 000007890 |
| 9876543 | 009876543 |
JS 數字前面補0
程式在處理資料時，很多時候都會用到Leading zero的概念，尤其一串編號要進行格式統整時，這時可以直接照下面這樣做。
// 數字、字串前補0
// code : 你輸入的資料(字串、數字)
// dataLength : 資料補0後的長度
function LeadingZero( code, dataLength){
var str = Array(10).join('0') + code;
return str.slice(0 - dataLength)
}
var str = 'abc123'
console.log(LeadingZero(str , 10))
// output : '0000abc123'如果你希望前面補的不是數字0 ，而是其他自訂的代號，可以直接把 join()中的0改成你要補的代號就好。
相容 IE 老系統
如果你用的系統很老，還在使用 IE 開啟，較早期的版本是沒有支援Join的，所以可以改寫成以下代碼：
// input: (123 , 8) , output : 00000123
function LeadingZero( code, dataLength){
var str = '0000000000' + code;
return str.slice(0 - dataLength);
}
var str = 'abc123'
console.log(LeadingZero(str , 10))
// output : '0000abc123'
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