javascript array if語句

Question

我正在嘗試爲一週的日子創建一個數組，但是我希望它在輸入的數字未分配給值時顯示錯誤。這是我所擁有的，但是在我的if語句後面寫着「今天是未定義的」。我認爲這是因爲陣列。有沒有辦法將if語句作爲數組的一部分，這樣只有一條消息彈出？

var dayNum=prompt('enter as number from 0 to 6', ' '); 
 
if (dayNum >='7'); 
 
{ 
 
alert("invalid input received, try again") 
 
} 
 
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "]; 
 
alert("Today is " + weekdays[dayNum]);

Answer 1

這是因爲分號在這裏沒有必要的;被終止聲明

if (dayNum >='7'); 

刪除分號。另外，如果輸入無效，您的第二種情況仍在運行。相反，將其放入一個else語句中。

var dayNum = Number(prompt('enter as number from 0 to 6', ' ')); 
 
if (isNaN(dayNum) || dayNum >= 7) 
 
{ 
 
    alert("invalid input received, try again") 
 
} else { 
 
    var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]; 
 
    alert("Today is " + weekdays[dayNum]); 
 
}

最後，它也是明智的，把它們當作一個之前將字符串轉換（這是迅速的返回類型）的號碼類型。在這種情況下，JavaScript知道數組的索引只能是數字，因此會自動嘗試爲您轉換類型。只要用戶鍵入一個有效的號碼，它就可以作爲索引而沒有問題。但是，字符串按字母順序進行比較。這意味着(string >= '7')可能並不總是按照您的預期行事：字母大於數字，但特殊字符較少。

Answer 2

有後if (dayNum >=7)

var dayNum=prompt('enter as number from 0 to 6', ' '); 
 

 

 
if (dayNum >=7) 
 
{ 
 
alert("invalid input received, try again") 
 
} 
 
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "]; 
 
alert("Today is " + weekdays[dayNum]); 
 
document.getElementById("demo").innerHTML = weekdays[dayNum];

Answer 3

您的代碼中幾乎沒有錯誤。首先是if條件之後的分號，另一個是使用字符串比較而不是數字。在if條件中刪除數字的引用。並且數組的索引也是錯誤的。你正在提供一個字符串。它應該是int類型。

<script> 
var dayNum=parseInt(prompt('enter as number from 0 to 6', ' ')); 
if (dayNum >= 7) 
{ 
alert("invalid input received, try again"); 

} 
else{ 
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "]; 
alert("Today is " + weekdays[dayNum]); 

} 
</script> 

Answer 4

其實，你會希望有輸入是1-7和有一個假的錯誤聲明，如果您使用0例如：

var weekdays = ["Error", "Sun", "Mon", ..., "Sat"]; 

那麼你將要做出一個循環將字符串轉換爲數字：

for(i=0;i<=7;i++){ 
    if(dayNum==i){ 
     dayNum=i; 
    } 
} 

這會給你一個整數而不是字符串，使用近似值。你釣代碼如下：

var dayNum=prompt('enter as number from 0 to 6', ' '); 
 
for(i=0;i<=7;i++){ 
 
    if(dayNum==i){ 
 
     dayNum=i; 
 
    } 
 
} 
 
if (dayNum <=7) 
 
{ 
 
    alert("invalid input received, try again"); 
 
} 
 
else if(dayNum === 0) 
 
{ 
 
    alert("invalid input received, try again"); 
 
} 
 
else 
 
{ 
 
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "]; 
 
alert("Today is " + weekdays[dayNum]); 
 
}

Answer 5

var dayNum=prompt('enter as number from 0 to 6', ' '); 
 
for(i=0;i<=7;i++){ 
 
    if(dayNum==i){ 
 
     dayNum=i; 
 
    } 
 
} 
 
if (dayNum >=7) 
 
{ 
 
    alert("invalid input received, try again"); 
 
} 
 
else if(dayNum === 0) 
 
{ 
 
    alert("invalid input received, try again"); 
 
} 
 
else 
 
{ 
 
var weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", " "]; 
 
alert("Today is " + weekdays[dayNum]); 
 
}