Erlang equivalent of javascript codePointAt?

You can use bit syntax pattern matching to skip the first N bytes and decode the first character from the remaining bytes as UTF-8:

1> CodePointAt = fun(Binary, Offset) ->
  <<_:Offset/binary, Char/utf8, _/binary>> = Binary,
  Char
end.

Test:

2> CodePointAt(<<"πr²"/utf8>>, 0).
960
3> CodePointAt(<<"πr²"/utf8>>, 1).
** exception error: no match of right hand side value <<207,128,114,194,178>>
4> CodePointAt(<<"πr²"/utf8>>, 2).
114
5> CodePointAt(<<"πr²"/utf8>>, 3).
178
6> CodePointAt(<<"πr²"/utf8>>, 4).
** exception error: no match of right hand side value <<207,128,114,194,178>>
7> CodePointAt(<<"πr²"/utf8>>, 5).
** exception error: no match of right hand side value <<207,128,114,194,178>>

As you can see, if the offset is not in a valid UTF-8 character boundary, the function will throw an error. You can handle that differently using a case expression if needed.

First, remember that only binary strings are using UTF-8 in Erlang. Plain double-quote strings are already just lists of code points (much like UTF-32). The unicode:chardata() type represents both of these kinds of strings, including mixed lists like ["Hello", $\s, [<<"Filip"/utf8>>, $!]]. You can use unicode:characters_to_list(Chardata) or unicode:characters_to_binary(Chardata) to get a flattened version to work with if needed.

Meanwhile, the JS codePointAt function works on UTF-16 encoded strings, which is what JavaScript uses. Note that the index in this case is not a byte position, but the index of the 16-bit units of the encoding. And UTF-16 is also a variable length encoding: code points that need more than 16 bits use a kind of escape sequence called "surrogate pairs" - for example emojis like - so if such characters can occur, the index is misleading: in "az" (in JavaScript), the a is at 0, but the z is not at 2 but at 3.

What you want is probably what's called the "grapheme clusters" - those that look like a single thing when printed (see the docs for Erlang's string module: https://www.erlang.org/doc/man/string.html). And you can't really use numerical indexes to dig the grapheme clusters out from a string - you need to iterate over the string from the start, getting them out one at a time. This can be done with string:next_grapheme(Chardata) (see https://www.erlang.org/doc/man/string.html#next_grapheme-1) or if you for some reason really need to index them numerically, you could insert the individual cluster substrings in an array (see https://www.erlang.org/doc/man/array.html). For example: array:from_list(string:to_graphemes(Chardata)).