Posts about REDUCE.
REDUCE is the most powerful of the functional array functions and the most overlooked. The lambda which means you can accumulate strings, and replace calculations that used to need VBA.
Joining a column of names or tags into one comma-separated string used to need TEXTJOIN or a helper column. REDUCE does it in one formula, and lets you transform each item before it gets concatenated.
LAMBDA, MAP, REDUCE, SCAN, BYROW. These are functional programming primitives. Excel is shipping them one by one, and most users are still writing nested IFs. The spreadsheet you learned in 2015 has been quietly replaced by something much more powerful.