Why does ++[[]][+[]]+[+[]] return the string “10”?

Question

This is valid and returns the string \"10\" in JavaScript (more examples here):

Answer 1

+[] evaluates to 0 [...] then summing (+ operation) it with anything converts array content to its string representation consisting of elements joined with comma.

Anything other like taking index of array (have grater priority than + operation) is ordinal and is nothing interesting.