It’s important that a recursive function has a stopping condition; without it, it will keep
recursing until the JavaScript interpreter decides the call stack is too deep (which will
cause your program to crash). In our
findNeedle
function, we have two stopping
conditions: we stop because we found the needle, or we stop because there is no hay‐
stack. Because we’re reducing the size of the haystack every time, it’s inevitable that
we will eventually reach one of these stopping conditions.
Let’s consider a more useful, time-honored example: finding the factorial of a num‐
ber. The factorial of a number is the number multiplied by every number up to it and
is denoted by an exclamation point after a number. So 4! would be 4 × 3 × 2 × 1 = 24.
Here’s how we would implement this as a recursive function:
function
fact
(
n
) {
if
(
n
===
1
)
return
1
;
return
n
*
fact
(
n
-
1
);
}
Here we have a stopping condition (
n === 1
), and every time we make the recursive
call, we reduce the number
n
by one. So eventually we’ll get to 1 (this function will fail
if you pass it
0
or a negative number, though of course we could add some error con‐
ditions to prevent that from happening).
Conclusion
If you have experience with other functional languages—like ML, Haskell, Clojure, or
F#—this chapter was probably a cakewalk for you. If not, it probably stretched your
mind a bit, and you might be reeling a little from the abstract possibility of functional
programming (believe me, when I first encountered these ideas, I certainly was). You
might be a little overwhelmed by the various ways you can accomplish the same
thing, and wondering which way is “better.” I’m afraid there’s no easy answer to that
question. Often it depends on the problem at hand: some problems strongly suggest a
certain technique. A lot also depends on you: what techniques resonate with you? If
you find the techniques presented in this chapter bewildering, I encourage you to
reread it a few times. The concepts here are extremely powerful, and the only way
you’ll know if they’re useful techniques for you is if you take some time to understand
them.
Conclusion | 197
