switch语句是否递归递归？

Question

I use switch statements now and again. Every so often I find myself wanting to use return statements inside of my functions. I was wondering if switch statements written this way were still tail-call optimized.

function misc(x) {
    switch(true){
        case x > 1:
            return misc(x-1);
            break;
        default:
            return;
    }
}

What would need to be changed to make case x > 1 tail call optimized. Is it possible to do with a switch statement or would I have to use something else like an if statement?

Note: I am aware that tail-call optimization isn't supported in JavaScript across most major browsers. I'm mostly just wondering if the above code is even compatible with tail-call optimization regardless if the current browser supports it or not.

Answer 1

The requirement for tail call optimisation in ES6 only depends on weather the function call is in the tail position - that is, weather the function is the last thing executed before returning.

In your case:

return misc(x-1);

puts misc() in the tail position. Therefore it can be tail-call optimised.

The case where that can't happen is if you were to do something like this:

return 1 + misc(x-1);

That's because it will be equivalent to:

var tmp = misc(x-1);
tmp = 1 + tmp;
return tmp;

Which means that misc() would not be in the tail position.

So it doesn't matter if the return is in an if or while or switch , as long as the function is in the tail position it can be tail-call optimised.