javascript switch语句/大小写表达式

Question

I am using a switch statement to search for undefined values to manually change it. but I am having trouble using boolean expressions as I would when using an if statement.

Like: if statemant

if(Item1 == undefined)
{
  item1 ="No";
}
else if (Item2 == undefined)
{
  item2 = "No";
}

etc..

I tried this with the switch statement :

 switch (array) {
 case (item1 == undefined):
 item1 = "No";
 console.log('item1 result', item1 );
 break;
 case item2 == undefined:
 item2 = "No";
 console.log('item2 result', item2 );
 break;
 default:

 }

It does not run through the switch statement, except for when I remove == undefined and only use item1 . then it works?

Answer 1

The switch cannot evaluate values of the array like that and that is why it does not run through the switch statement. You need to define which value of that array you want to switch.

Inside case statement you also cannot use expression, you have to use a value there as well.

So, if you are dead set on using switch for what you are trying to accomplish, you can do something like this:

item1 = array[1];
switch(item1) {
    case "undefined":
        // so on
    break;
}

But, based on your example you are probably trying to check if the values are set or not, for that if statements are still the best choice rather than switch.

$arr = []; // Your array
if(typeof $arr[0] == "undefined") {
    $arr[0] = "No";
}

Answer 2

switch can be applied only 1 variable as follows:

switch (array) {
 case 'undefined':
 item1 = "No";
 console.log('item1 result', item1 );
 break;
 case array.length:
 item2 = "No";
 console.log('item2 result', item2 );
 break;

 }

Answer 3

for(var i=0;i<array.length;i++){ 
    switch(array[i])  {
    case "undefined":
   //so on
      break;
  }
}

Answer 4

Useful when reading arguments of function.

Short and simple approach would be

item1 = (item1 == null)? item1 : 'no';
item2 = (item2 == null)? item2 : 'no';

If item is undefined or null it will assign value otherwise that particular item.

ps this is for checking null or undefined and better than using switch