使用Switch語句的JavaScript - 沒有得到預期的輸出

Question

我很新，並且有很多東西需要學習。我在這段JavaScript中使用了Switch語句，但是我沒有按照document.write行獲得預期的輸出。任何幫助，意見和建議表示讚賞。 - 謝謝！

  <script type="text/javascript"> 

      var myAge = Number(prompt("Enter your age", 30)); 
      myAge = parseInt(myAge); 

      switch (myAge) 
      { 
       case (myAge >= 0 && myAge <= 10): 
        document.write("myAge is between 0 and 10"); 
        break; 

       case (!(myAge >= 0 && myAge <=10)): 
        document.write ("myAge is NOT between 0 and 10 <br />"); 
        break; 

       case (myAge >= 80 || myAge <= 10): 
        document.write ("myAge is 80 or above OR 10 or below <br />"); 
        break; 

       case (myAge >= 30 && myAge <=39 || (myAge >= 80 && myAge <= 89)): 
        document.write("myAge is between 30 and 39 or myAge is between 80 and 89"); 
        break; 

       default: 
        document.write("You did not enter a number. Please enter a number."); 
        break; 
      }  

      document.write("<BR>Execution continues here"); 


     </script> 

這就是我剛纔用'if'寫的。

 <script type="text/javascript"> 

     var myAge = Number(prompt("Enter your age", 30)); 

     if (myAge >= 0 && myAge <= 10) 
     { 
      document.write ("myAge is between 0 and 10 <br />"); 
     } 

     if (!(myAge >= 0 && myAge <=10)) 
     { 
      document.write ("myAge is NOT between 0 and 10 <br />"); 
     } 

     if (myAge >= 80 || myAge <= 10) 
     { 
      document.write ("myAge is 80 or above OR 10 or below <br />"); 
     } 

     if (myAge >= 30 && myAge <=39 || (myAge >= 80 && myAge <= 89)) 
     { 
      document.write("myAge is between 30 and 39 or myAge is between 80 and 89"); 
     } 


    </script> 

這是使用'開關'，我必須參考的示例代碼片段。

<script type="text/javascript"> 

var secretNumber = prompt("Pick a number between 1 and 5:", ""); 
secretNumber = parseInt(secretNumber); 

switch (secretNumber) 
{ 
case 1: 
    document.write("Too low!"); 
    break; 

case 2: 
    document.write("Too low!"); 
    break; 

case 3: 
    document.write("You guessed the secret number!"); 
    break; 

case 4: 
    document.write("Too high!"); 
    break; 

case 5: 
    document.write("Too high!"); 
    break; 

default: 
    document.write("You did not enter a number between 1 and 5."); 
    break; 
} 
document.write("<BR>Execution continues here"); 

</script> 

Answer 1

您需要重新考慮case表達式中的邏輯。

讓我在你的代碼替換，來說明錯誤

var test = myAge >= 0 && myAge <= 10; 
if (test) { 
    ... 
} else if (!test) { 
    ... 
} else { 
    // WILL NEVER BE RUN 
} 

如果你想使用你有一個switch語句來了解你使用的數據背後的類型。

true : Boolean 
false : Boolean 
9 : Number 
1.0 : Number 

在下面的例子， v必須具有相同的類型和value1value2。

switch(v) 
{ 
    case (value1): 
    ...  
    case (value2): 
    ... 
} 

問題是以下類型在您的示例中不匹配。

v = Number 
value1 = Boolean 
value2 = Boolean 

Answer 2

對於這樣的條件下，使用if：

if (myAge >= 0 && myAge <= 10){ 
    document.write("myAge is between 0 and 10"); 

}else if (!(myAge >= 0 && myAge <=10)){ 
    //... 
} 

switch是多個正是價值觀，這不是你的情況：

switch(myAge){ 
    case 0: 
     document.write("myAge is *exactly* 0"); 
     break; 
    //... 
} 

Answer 3

你可以讓一個開關，將只評估真實的陳述，它填充找到第一個真正的陳述（案例），如果有的話，或者它會顯示默認。它將使用條件邏輯來工作。所以，你的代碼工作剛剛添加的，而不是myAge

var myAge = Number(prompt("Enter your age", 30)); 

switch (true) { 
    case (myAge >= 0 && myAge <= 10): 
     document.write("myAge is between 0 and 10"); 
     break; 

    case (!(myAge >= 0 && myAge <= 10)): 
     document.write("myAge is NOT between 0 and 10 <br />"); 
     break; 

    case (myAge >= 80 || myAge <= 10): 
     document.write("myAge is 80 or above OR 10 or below <br />"); 
     break; 

    case (myAge >= 30 && myAge <= 39 || (myAge >= 80 && myAge <= 89)): 
     document.write("myAge is between 30 and 39 or myAge is between 80 and 89"); 
     break; 

    default: 
     document.write("You did not enter a number. Please enter a number."); 
     break; 
} 

剛一說明，使用開關一樣，這是一個性能矯枉過正真。使用if/else語句會更好/更快。

您可以在這裏閱讀關於開關Switch的更多信息，並在底部可以看到使用此方法（方法二）。