[rhn_mk1]

"asdf".indexOf("as") is 0

but

"asdf"[0] is NOT "as".

so there can be no expectation that ""[0] is "".

Those two operations are not related to each other. It's more intuitive if you treat .indexOf() as .startOf(). Then "asdf"[0..x] is "as" for x=2, and ""[0..x] is "" for x=0.