Flexible is a double-edged sword. `querySelector` is bringing in the added complexity of selector syntax, which is more stuff to think about that isn't relevant to what you're trying to do.
For example now you have to worry about whether there are any characters in the ID that need escaping in a selector (eg `.`), something that may not be easy to verify when formatting an ID out of variables.
So I'd suggest preferring getElementById for its directness, rather than for micro-optimisation reasons.
(In principle the same should be true for getElementsByClassName, but the live NodeLists returned by that method are a trap for the unwary, so neither option is ideal.)
For example now you have to worry about whether there are any characters in the ID that need escaping in a selector (eg `.`), something that may not be easy to verify when formatting an ID out of variables.
So I'd suggest preferring getElementById for its directness, rather than for micro-optimisation reasons.
(In principle the same should be true for getElementsByClassName, but the live NodeLists returned by that method are a trap for the unwary, so neither option is ideal.)