I saw that they were documented together here. Are they the same thing? Why does Ruby have so many aliases (such as map/collect for arrays)? Thanks a lot.
Yes, and it's also called fold in many other programming languages and in Mathematics. Ruby aliases a lot in order to be intuitive to programmers with different backgrounds. If you want to use #length on an Array, you can. If you want to use #size, that's fine too!
More recent versions of the documentation of Enumerable#reduce specify it explicitly:
The
injectandreducemethods are aliases. There is no performance benefit to either.
来源：https://stackoverflow.com/questions/13813243/is-inject-the-same-thing-as-reduce-in-ruby