表單元素點(diǎn)擊事件改到script標(biāo)簽中就不能執(zhí)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(biāo)簽中加上numCount()就報(bào)錯(cuò)，
提示cannot set property 'value' of null
2016-11-02
因?yàn)轫撁婕虞d順序從上到下，當(dāng)執(zhí)行numCount函數(shù)，加載到document.getElementById('txt').value=num這一句時(shí)，body里面的代碼還未獲取，因此會(huì)報(bào)錯(cuò)，只要將整個(gè)<script>標(biāo)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(shí)立刻執(zhí)行numCount() function?numCount(){ ?document.getElementById('txt').value=num; ?num=num+1; ?setTimeout("numCount()",1000); ?} </script> </body> </html>