This might be simpler:

// Hack around mangling of left square bracket by PerlMonks.

function get_item(a, i) {
   return eval("a\x5Bi]");
}

function set_item(a, i, v) {
   return eval("a\x5Bi] = v");
}
[download]

Usage:

var a = new Array(5);
set_item(a, 2, "Hello World\n");
alert(get_item(a, 2));
[download]

Update: Never mind. New useful escapes documented here.