Map.merge() – One method to rule them all

And also rather obscure and rarely used.can be explained as follows: it either puts new value under the given key (if absent) or updates existing key with a given value (UPSERT). code was quite messy and the essence was lost in implementation details:However, it works and for given input produces desired output:OK, but let’s try to refactor it to avoid conditional logic:That’s nice!is a necessary evil, otherwise, the code breaks on the first occurrence of a previously unknown word. Imagine you have an account operation (constructor, getters and other useful properties omitted):And a bunch of operations for different accounts:We would like to compute balance (total over operations’ amounts) for each account.

Source: www.nurkiewicz.com