Updating String/array Items

I'm not entirely familiar with the norms and standard libraries of
JavaScript so if the answer to this is yesterday's news, please ignore.

I'm trying to write a simple text formatting function to make headings
proper case -- i.e. first letter of words capitalized.

I first tried this...

function inProperCase(s)
{
var result, i, k;
i = 0; result = s.toLowerCase();
while ((k = result.indexOf(' ', i)) -1) {
if (k - i 0) {
result[i] = result[i].toUpperCase();
}
i = k + 1;
}
if (result.length - i 1) {
result[i] = result[i].toUpperCase();
}
return result;
}

...but it didn't work. To my amazement, you cannot set a character in a
string by accessing that character with an index, like so:

s[i] = ch;

Must be because JavaScript strings are "Immutable" or "EtchedInStone" or
something. (Just kidding, no flames, please. I'm sure it is more
efficient.)

The only thing I could come up with was this (with some details, like the
"compound" while predicate, modified into a more readable form after being
called a "C-bigot" by a friend :)) which seems pretty heavy-handed with all
the string slicing and concatting:

function inProperCase(s)
{
var result = s.toLowerCase(),
i = 0, k = result.indexOf(' ', i);
while (k -1) {
if (k - i 0) {
result = result.substring(0, i) +
result[i].toUpperCase() +
result.substring(i+1, result.length);
}
i = k + 1;
k = result.indexOf(' ', i);
}
if (result.length - i 1) {
result = result.substring(0, i) +
result[i].toUpperCase() +
result.substring(i+1, result.length);
}
return result;
}

You call this as...

inProperCase(" madam i'm adam ");

...and you get...

" Madam I'm Adam "

Any suggestions for making this a bit more elegant?

BTW You may point me to a "library" with a super-efficient implementation
of this function, but that is not the point. My issue is, how do we update
single characters in a string (or single items in an array) in JavaScript?
Is it something like...

s.wouldYouCareToUpdateThisChar(i, ch); // :D (just kidding, calm
down...)

-or-

s.setCharAt(i, ch); ...