In VB if I wanted to bine a case statment it would look like this

Select (somevalue)
  Case 1, 2, 3:
    Do Something
End Select

In C# and Javascript

switch (someValue) {
  case 1:
  case 2:
  case 3:
     //dosomething
     break;
}

However this runs without errors in Javascript

switch (someValue) {
  case 1, 2, 3:
     break;
}

But does not do what is expected. What is it actually doing?

The reason I ask is because if I hover over the 1, 2, or 3 in firebug it specifies the watch as false. So clearly the code is evaluating but what is it evaluating.

In VB if I wanted to bine a case statment it would look like this

Select (somevalue)
  Case 1, 2, 3:
    Do Something
End Select

In C# and Javascript

switch (someValue) {
  case 1:
  case 2:
  case 3:
     //dosomething
     break;
}

However this runs without errors in Javascript

switch (someValue) {
  case 1, 2, 3:
     break;
}

But does not do what is expected. What is it actually doing?

The reason I ask is because if I hover over the 1, 2, or 3 in firebug it specifies the watch as false. So clearly the code is evaluating but what is it evaluating.

• c#
• javascript
• vbscript

• 2 1, 2, 3 is an expression that evaluates to 3. – Gabe Commented Dec 15, 2010 at 19:42

3 Answers 3

The Javascript ma operator evaluates both its operands in left to right order, returning the rightmost. So, you essentially wrote

switch (someValue) {
    case 3:
        break;
}

You could also use:

switch(true) {
  case (somevalue <= 3): /* action if <= 3 */ break;
  case (somevalue <= 6): /* action if <= 6 */ break;
  //[etc]
  default: 'no action' 
}

Just for fun: to be able to pare a number against multiple values a Number extension may e in handy:

Number.prototype.In = function(){
    var i = -1, args = arguments;
    while (++i<args.length){
         //use float for all numbers
         if (parseFloat(this) === parseFloat(args[i])){
         return true;
        }
    }
    return false;
};

and then your switch bees:

switch(true) {
  case somevalue.In(1,2,3):   /* action if 1,2,3 */ break;
  case somevalue.In(6,10,14): /* action if 6,10,14 */ break;
  //[etc]
  default: 'no action' 
}

See also O'Reilly - chapter on switch

MDarwi beat me to it, nonetheless he nailed it,

<script type="text/javascript">
var x = 5;
switch (x)
{
  case 5, 6, 7:
    document.write("<b>This should work on 5, 6 or 7.</b>");
    break;
  case 0:
    document.write("<b>This should work on 0.</b>");
    break;
}
</script>

..writes the first case only when x == 7.