javascript錯誤：函數未定義

Question

我試圖找到我的代碼無法正常工作的原因，並且chrome帶回了這個錯誤：Uncaught ReferenceError：calcMPG未定義...有人能發現我的錯誤嗎？（絕望）

<script type="text/javascript">
    function calcMPG() {
        document.calc.startingMileage.value = startMileage;
        document.calc.endingMileage.value = endMileage;
        document.calc.gallonsUsed.value = gallUsed;
        var MPG = (endMileage - startMileage) / gallUsed;
        if (isNAN(startMileage) || isNAN(endMileage) || isNAN(gallUsed)) alert('please type in numbers only!');
        else document.calc.milesPerGalon.value = MPG;
    }
</script>

<form name="calc">Starting mileage:
    <input type="text" value="0" name="startingMileage" onchange="calcMPG()">
    <br>Ending mileage:
    <input type="text" value="0" name="endingMileage" onchange="calcMPG()">
    <br>Gallons used:
    <input type="text" value="0" name="gallonsUsed" onchange="calcMPG()">
    <br>Miles per galon:
    <input type="text" value="0" name="milesPerGalon">
</form>

Answer 1

您的聲明有誤，請更正。

var startMileage = document.calc.startingMileage.value ;
var endMileage =  document.calc.endingMileage.value;
var gallUsed =  document.calc.gallonsUsed.value;

Answer 2

startMileage

而你的其他右側引用並不意味着什么，它們是未定義的變量。

為您的輸入提供一個id並從中獲取值，如下所示：

var startingMileage = document.getElementById('startingMileage').value;

Answer 3

我不知道你為什么那樣做，但試試這個：

<script type="text/javascript">
  function calcMPG(){
    var startMileage  = document.calc.startingMileage.value,
        endMileage    = document.calc.endingMileage.value,
        gallUsed      = document.calc.gallonsUsed.value,
        MPG           = (endMileage - startMileage) / gallUsed;

      if(isNaN(startMileage) || isNaN(endMileage) || isNaN(gallUsed)){
        alert('please type in numbers only!');
      } else {
        document.calc.milesPerGalon.value = MPG;
      }
    }
</script>
<form name="calc">
    Starting mileage:<input type="text" value="0" name="startingMileage" onchange="calcMPG()"><br>
    Ending mileage:<input type="text" value="0" name="endingMileage" onchange="calcMPG()"><br>
    Gallons used:<input type="text" value="0" name="gallonsUsed" onchange="calcMPG()"><br>
    Miles per galon:<input type="text" value="0" name="milesPerGalon">
</form>

Answer 4

點擊這里小提琴

 function calcMPG(){
        var startMileage = document.calc.startingMileage.value;
        var endMileage = document.calc.endingMileage.value;
        var gallUsed = document.calc.gallonsUsed.value;

        var MPG = (endMileage - startMileage) / gallUsed; 

        if(isNaN(startMileage) || isNaN(endMileage) || isNaN(gallUsed))
            alert('please type in numbers only!');
        else 
            document.calc.milesPerGalon.value = MPG;
    }