Returning value from a javascript callback

Question

I'm having trouble with returning a value from a callback. I'm using https://github.com/transducer/cljs-iota which is a wrapper arounf the IOTA javascript library.

My code is :

(defn find-transactions 
    "search transactions associated with an address and return a vector of hashes" 
    []
            (println "finding transactions")
            (iota-api/find-transactions eff.views/iota-instance {:addresses (list comment-address)} find-callback)
          )

And the callback function is:

(defn find-callback [err hashes]
          (prn  "transactions found" hashes)
          hashes
    )

If I do

(.log js/console "result is" (find-transactions))

The callback prints to the console :

"transactions found" ["JW9QTNHDLGQYKGHLYHWHYRKTTEVUVBZSHNMLCDLBWVYPROKTCDRAMFGXMIQCEM9YIRXUSZJTEKG999999" "UTJ9YBLCE9RJIERRBX9HFANUYFALKZJHAPXWHQKS9KQJEKUQOTLDTSCKAM9KIFPNRXSEJCHTUJT999999" "PYWIZA9JAIPDEQBTMHSXVSKOLYGSNSMVQKOBT9WXHEFLFYCKEVPPRMGQXCBMACKSGJDQKTCUZXH999999" "XJPCFXWELTKSNCCZQM9QNTFMSRMNAEJ9WJWQERLMDSHQTGGQQHKZRZRANGRRXXTGSUNBSDFDBAC999999" "WVQIPEZZUPYODYFO9KYAIEQYPTGIMKDXGRPGF9ADDVKGGMRFKKASXQLKATFDIJJPWLOPSPPYURDSA9999" "QYKZWELJYAMWPITSTZSXLFFERSGRPBKIOCHSE9KLENRXNBXSGSLBEYW9JUVJYP9QWBJJFKMSUDA999999"]

which is the result I want, but I get "result is" undefined.

I've been blocked on this for the whole day and I don't know why I can't return the result (if I try to return for example "hello" it doesn't work either), and my function works because I get the result back but cant return it...

If anyone has a suggestion...

Justin L. · Accepted Answer · 2018-06-26 00:18:27Z

I already answered on reddit but I'll add an answer here for posterity.

In callback style programming, you don't get a return. The callback needs to cause an effect somewhere, for example, by mutating an atom, putting something into a core.async channel, or resolving a promise (perhaps with the assistance of the promesa library. I prefer promises, but ymmv.

(defn find-transactions
  "search transactions associated with an address and return a promise for vector of hashes" 
  []
  (println "finding transactions")
  (promesa/promise
    (fn [resolve _reject]
      (iota-api/find-transactions 
        eff.views/iota-instance 
        {:addresses (list comment-address)} 
        (fn [error hashes]
          (if (error)
            (reject error)
            (resolve hashes)))))))

(-> (find-transactions)
    (promesa/then (fn [result]
                    ; do something here with result
                    ))
    (promesa/then (fn [error]
                    ; do something with error
                    )))

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