is switch(true) {… valid javascript?

Question

I recently came across code where a switch statement seemed reversed with the answer (boolean) in the switch and the expressions in the case. The code ran fine as intended b

Answer 1

It is valid.

The code

switch(f0()) {
    case f1(): ..; 
    case f2(): ..;
    default: dflt;
}

where fX() represents an arbitrary expression (a function invocation is used to show forced evaluation) can be approximately re-written as

for (;;) { // for "break"
    var _x = f0()
    if (_x === f1()) { .. }
    if (_x === f2()) { .. }
    dflt;
    break;
}

That is, the expression in the case is evaluated and then compared with the expression in the switch. (This is a sharp divergence from languages like C or Java that require constant values in the case expressions.)

Of course, the break will "exit the switch" - as opposed to the standard fall-through semantics - and as such, where true is the expression supplied to switch, the posted example is semantically equivalent to the if/else if as shown by aefxx.