I just ran into a bit of confusion today, "string".indexOf(''); always returns 0, but I would expect -1 (for false); inversely, "string".lastIndexOf(''); always returns 6
lastIndexOf is easier to understand, since string is 6 letters long ("string".length, being zero-indexed returns 5) but I don't see anywhere in the ECMAscript spec (5.1 or 6.0) that describes why '' would be treated like a word/character boundary
What, exactly, is going on here?
The spec says:
Return the smallest possible integer k not smaller than start such that k+searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the character at position k+j of S is the same as the character at position j of searchStr; but if there is no such integer k, then return the value -1.
That condition is fulfilled at position 0 because of vacuous truth: since you are searching the empty string, any statement you can think of will hold for every character, because it has no characters.
More formally, for any statement P, if S = ∅, P(x) holds ∀ x ∈ S.