Why can't I call seq functions in a sequence generated by js->clj?

Question

Although I can get turn a simple js object into a clojure object with something like;

(-> "{a: 2, b: 3}" js* js->clj)

I'm apparently not being able to do so with a particular object, goog.events.BrowserEvent, in a handler function like:

(defn handle-click [e]
  ...
  (-> e .-evt js->clj keys) ;; <-------------
  ...

The function does get applied, but the resulting object doesn't respond to sequence functions like countor first, although I can fetch items using aget. The error message I get, in chrome's console, is;

Uncaught Error: No protocol
method ISeqable.-seq defined for type object: [object Object]

Why is this happening? Shouldn't js->clj work with all objects?

How can I fix this?

Thanks!

Rodrigo Taboada · Accepted Answer · 2013-05-07 04:28:44Z

4

The js->clj only changes something that is exactly a JavaScript object (it is implemented using instance? instead of isa?, and with good reasons), when you pass a descendant of js\Object js->clj returns the same object. aget (and aset) works because it compiles down to the object[field-name] syntax on JavaScript.

You can extend the ISeq protocol (or any other protocol) to the goog.events.BrowserEvent and all functions that works with ISeq will work with goog.events.BrowserEvent. There is a talk by Chris Houser where he showed how to extend a bunch of protocols to a goog Map. I recommend watching the whole talk, but the part that are relevant to your question begins at approximately 14 minutes.

edited May 7, 2013 at 4:28

answered Apr 21, 2013 at 16:34

Rodrigo Taboada

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konr · Accepted Answer · 2013-04-17 01:20:31Z

1

First, I found out functions in google closure to get the keys and values of an object:

 (defn goog-hash-map [object]
   (zipmap (goog.object/getKeys object) (goog.object/getValues object)))

Then, by studying the source of cljs.core, I realized all I had to do was to extend the IEncodeClojure interface with it:

 (extend-protocol IEncodeClojure
   goog.events.BrowserEvent
   (-js->clj
    ([x {:keys [keywordize-keys] :as options}]
       (let [keyfn (if keywordize-keys keyword str)]
         (zipmap (map keyfn (gobj/getKeys x)) (gobj/getValues x))))
    ([x] (-js->cljs x {:keywordize-keys false}))))

The original code doesn't work on this object, because its type must be exactly Object. I tried to change the comparison function to instance?, ie,

(instance? x js/Object) (into {} (for [k (js-keys x)]
                                      [(keyfn k) (thisfn (aget x k))]))

but that didn't work either, wielding the following error, which made me settle for the previous approach.

Uncaught TypeError: Expecting a function in instanceof check, 
but got function Object() { [native code] }`.

answered Apr 17, 2013 at 1:20

konr

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1 Comment

Rodrigo Taboada Over a year ago

(instance? x js/Object) doesn't work because the first parameter to instance? should be a class. You should use (isa? x js/Object) instead.

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