Message from JavaScript discussions

November 2017

— Can you explain? I am really interested


function useOldEntry(oldEntry) {
entryEmitter.once('entry', function (entry) {
// access to oldEntry and entry variables
return useOldEntry(entry);

useOldEntry(null) // no old entry on startup

— This way, for each iteration, you have access to current and previous version of an "entry"

— Caveats:
.once may be slow
useOldEntry HAS to be called synchronously within the callback to avoid possibility of skipping entries

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— Pros:
Pure functional ish
Gets the job done

— Alright maybe I need to see this in action in my actual code to see the picture

— But, ehh, monads may be nicer

— Because, yes the concept seems doable

— But I can't see the spot to fit It

— I have an actual nodejs example

— A HTTP server that increments a counter without modifying state


function count(i) {
server.once('request', function (req, res) {
return count(i + 1);


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