Why don't OR operators work in Switch statements

Question

Java Switch Statement - Is "or"/"and" possible?

There are quite a few answers like the above that give good alternatives to using this erroneous syntax in my example but I haven't read an explanation to why it doesn't work:

const switchExample = (val) => {
    switch(val) {
        case 'a' || 'b' || 'c':
        return 'first 3'
        break;
        case 'd':
        return 'fourth'
        break;
        default:
        return 'default'
    }
}

Invoking with 'b' or 'c' as a param will return the default. Whereas 'a' will return (as expected) 'first 3'. I was expecting that if 'b' was truthy, then the first case would evaluate to true and return 'first 3' but this ins't the case. Can somebody explain why?

Because that's not the syntax of the language to combine cases? Or do you want the technical details about what happens? — Some programmer dude, Apr 30 '20 at 08:09
`('a' || 'b' || 'c') === 'a'` (there are a few questions on SO that cover this, e.g. https://stackoverflow.com/questions/2966430/why-does-javascripts-or-return-a-value-other-than-true-false). The `switch` is treating all of the `case` expressions the same: as values to compare against. — Ry-, Apr 30 '20 at 08:10
Thanks - no need for technical details - ('a' || 'b' || 'c') === 'a' clears this up for me - I need to read up on logic rather than the switch statements per se. — BBrow, Apr 30 '20 at 08:38

score 0 · Answer 1 · answered Apr 30 '20 at 08:09

It does not work, because the first truthy value is taken by using logical OR || for a strict comparison.

Alternatively, you could use a fall through approach with having three cases in a line, like

const
    switchExample = (val) => {
        switch(val) {
            case 'a':
            case 'b':
            case 'c':
                return 'first 3'; // no break required, because return ends the function
            case 'd':
                return 'fourth';
            default:
                return 'default';
        }
    }

score 0 · Accepted Answer · answered Apr 30 '20 at 08:11

Here

    switch(val) {
        case 'a' || 'b' || 'c':

the value of the expression 'a' || 'b' || 'c' will be first evaluated (it's 'a'), and then the switch will examine whether val fits the case.

You need to do the alternative this way:

    switch(val) {
        case 'a':
        case 'b':
        case 'c':
            return 'first 3';
        case 'd':
// ...

Why don't OR operators work in Switch statements

2 Answers2