                        課程
                    
                        /前端開發
                        
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                        /JavaScript進階篇

為什么我的結果會出現NaN

<!DOCTYPE html>
<html>

?<head>
??<meta charset="UTF-8">
??<title></title>
??<script type="text/javascript">
???function celement() {
????var value1 = document.getElementById("va1").value;
????var value2 = document.getElementById("va2").value;
????var tich = document.getElementById("select").value;
????var result;
????//????if(tich=="+"){
????//?????result = parseFloat("value1") + parseFloat("value2");
????//????}
????//????else if(tich=="-"){
????//?????result = parseFloat("value1") - parseFloat("value2");
????//????}
????//????else if(tich=="*"){
????//?????result = parseFloat("value1") * parseFloat("value2");
????//????}
????//????else{
????//?????result = parseFloat("value1") / parseFloat("value2");
????//????}
????switch (tich) {
?????case "+":
??????result = parseFloat("value1") + parseFloat("value2");
??????break;
?????case "-":
??????result = parseFloat("value1") - parseFloat("value2");
??????break;
?????case "*":
??????result = parseFloat("value1") * parseFloat("value2");
??????break;
?????default:
??????result = parseFloat("value1") / parseFloat("value2");
??????break;
????}
????document.getElementById("incon").value = result;
???}
??</script>
?</head>

?<body>
??
??<br> 計算器：
??<br>
??<input type="text" name="" id="va1" value="" />
??<select name="" id="select">
??? ?<option value="+">+</option>
??? ?<option value="-">-</option>
??? ?<option value="*">*</option>
??? ?<option value="/">/</option>
??? ?</select>
??<input type="text" name="" id="va2" value="" />
??<input type="button" name="" id="" value="=" onclick="celement()" />
??<input type="text" name="" id="incon" value="" />
?</body>

</html>