Convert integer into its character equivalent, where 0 => a, 1 => b, etc

A simple answer would be (26 characters):

String.fromCharCode(97+n);

If space is precious you could do the following (20 characters):

(10+n).toString(36);

Think about what you could do with all those extra bytes!

How this works is you convert the number to base 36, so you have the following characters:

0123456789abcdefghijklmnopqrstuvwxyz
^         ^
n        n+10

By offsetting by 10 the characters start at a instead of 0.

Not entirely sure about how fast running the two different examples client-side would compare though.

Will be more portable in case of extending to other alphabets:

char='abcdefghijklmnopqrstuvwxyz'[code]

or, to be more compatible (with our beloved IE):

char='abcdefghijklmnopqrstuvwxyz'.charAt(code);

Assuming you want lower case letters:

var chr = String.fromCharCode(97 + n); // where n is 0, 1, 2 ...

97 is the ASCII code for lower case 'a'. If you want uppercase letters, replace 97 with 65 (uppercase 'A'). Note that if n > 25, you will get out of the range of letters.

If you don't mind getting multi-character strings back, you can support arbitrary positive indices:

function idOf(i) {
  return (
    (i >= 26 ? idOf(((i / 26) >> 0) - 1) : "") +
    "abcdefghijklmnopqrstuvwxyz"[i % 26 >> 0]
  );
}

[0, 1, 25, 26, 27, 701, 702, 703].map(idOf);
// ['a', 'b', 'z', 'aa', 'ab', 'zz', 'aaa', 'aab']

(Not thoroughly tested for precision errors :)