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表單元素點擊事件改到script標簽中就不能執行？？

<!DOCTYPE?HTML>
<html>
<head>
<script?type="text/javascript">
var?num=0;
//?numCount();
function?numCount(){
?document.getElementById('txt').value=num;
?num=num+1;
?setTimeout("numCount()",1000);
?}
</script>
</head>
<body>
<form>
<input?type="text"?id="txt"?/>
<input?type="button"?value="Start"?onclick="numCount()"?/>
</form>
</body>
</html>

為什么我把onclick="numCount()"刪除，然后在script標簽中加上numCount()就報錯，

提示cannot set property 'value' of null

臨風流煙

2016-11-02

源自：JavaScript進階篇 8-5

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1 回答

stone310
2016-11-02

因為頁面加載順序從上到下，當執行numCount函數，加載到document.getElementById('txt').value=num這一句時，body里面的代碼還未獲取，因此會報錯，只要將整個<script>標簽放到</body>上方即可，

<!DOCTYPE?HTML>
<html>
<head>
</head>
<body>
<form>
<input?type="text"?id="txt"?/>
<input?type="button"?value="Start"??/>???<!--刪除了onclick="numCount()"-->
</form>
<script?type="text/javascript">
var?num=0;
?numCount();????????????????????????????//頁面加載時立刻執行numCount()
function?numCount(){
?document.getElementById('txt').value=num;
?num=num+1;
?setTimeout("numCount()",1000);
?}
</script>
</body>
</html>