pragma thread

 
function fib(i, last = 0) {
    if (i < 0) return undefined
    if (i <=1) return i
    return fib(i - 1) + fib(i - 2)
}

fib(42)
            

 
let thr = _thr().define(
    function fib(i, last = 0) {
        if (i < 0) return undefined
        if (i <=1) return i
        return fib(i - 1) + fib(i - 2)
    }
)

thr.fib(42)