Expression inside switch case statement

Question

I'm trying to create a switch statement but I can't seem to be able to use an expression that gets evaluated (rather than a set string/integer). I can easily do this with if statements but case should hopefully be faster.

I'm trying the following

function reward(amount) {     var $reward = $("#reward");     switch (amount) {         case (amount >= 7500 && amount < 10000):             $reward.text("Play Station 3");             break;         case (amount >= 10000 && amount < 15000):             $reward.text("XBOX 360");             break;         case (amount >= 15000):             $reward.text("iMac");             break;         default:             $reward.text("No reward");             break;     } } 

Am i missing something obvious or is this not possible? Google hasn't been friendly in this case.

Any help/pointers appreciated

M

Answer 1

amount is a number, but the expressions in the case clauses only evaluate to booleans; the values will never match.

You could always do

switch (true) {   case (amount >= 7500 && amount < 10000):     // Code     break;   case (amount >= 10000 && amount < 15000):     // Code     break;   // etc. } 

It works because the value being matched is now the boolean true, so the code under the first case clause with an expression that evaluates to true will be executed.

It’s kinda “tricky”, I guess, but I see nothing wrong with using it. A simple if–else statement would probably be more concise, and you’d not have to worry about accidental fall-through. But there it is anyway.

Answer 2

@MooGoo's switch (true) will give you a Weird condition error in jsLint, so let's get a little more creative in case that's an issue, and, I think, increase readability a touch.

So we're not evaluating if each case is true or false; we're comparing if that case's value is equal to our switch term. So let's take advantage of that by throwing a shorthand if into our case statement and return our original switch term if the condition's true.

I'm also including a sort of real world example, where you want to have two "defaults" -- one if your term is outside of your "important" range in the positive direction, and another in case you're in the negative direction.

Key phrase: case (x > 0 ? x : null):

"If my term, x, is greater than zero, return x so that x === x and I take the case branch."

http://jsfiddle.net/rufwork/upGH6/1/

/*global document*/ /*jslint evil:true*/ var x = 10;  switch (x) {     case (x > 0 ? x : null):         document.write('ha ha ha!  I fooled switch AND jsLint!  Muhahahahaha!');         break;     case 0:         document.write('zero is nothing.');         break;     case -1:         document.write('low');         break;     case -2:         document.write('lower');         break;     case -3:          document.write('lowest I care about');         break;     default: // anything lower than -3.         document.write('TOO LOW!!!! (unless you cheated and didn\'t use an int)'); } document.write('<br>done.'); 

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Quick reply to @Sv443:

Do notice that the default: switch says, "unless you cheated and didn't use an int" and that short circuiting requires x === x when you return x.

But your point is a useful reminder that NaN is the only case where short circuiting can't apply.

That is, x must == x to short circuit in switch and, as MDN tells us, "NaN, and only NaN, will compare unequal to itself" (double or triple =).

That also means that switching on a NaN value (and only a NaN value) will always hit default in ANY switch because you can't match its value.

Here's the full quote from MDN:

NaN compares unequal (via ==, !=, ===, and !==) to any other value -- including to another NaN value. Use Number.isNaN() or isNaN() to most clearly determine whether a value is NaN. Or perform a self-comparison: NaN, and only NaN, will compare unequal to itself.

You could change the default logic to check what you have:

isNaN(x) ? document.write ('nan') : document.write('TOO LOW!!!! ...)');

Or you even could go full hipster like MDN suggests (but please don't ;^D):

x !== x ? document.write ('nan') : document.write('TOO LOW!!!! ...)');