Idiomatically accessing json objects with clojurescript

Asked 22/3, 2012 at 10:55 Answered 29/1, 2018 at 13:34

Anyone have any docs for idiomatic clojurescript for access a javascript object (returned as json, essentially a hash)?

I have a JSON object returned via an AJAX request:

{
  list: [1,2,3,4,5],
  blah: "vtha",
  o: { answer: 42 }
}

How do I access these fields using clojurescript?

I can do:

(.-list data)

But how does this work when I have nested values and objects?

(.-answer (.-o data))

The above seems to be quite clumsy, especially given the nice js syntax of: data.o.answer.

What is the idiomatic way of accessing json objects with clojurescript?

Note:

I realised that I can actually refer to elements using JS syntax, which is quite handy actually. So the following will work correctly:

(str data.o.answer)

Incurious answered 22/3, 2012 at 10:55 Comment(3)

I assume in your actual setup, you get proper JSON? – Itemize 22/3, 2012 at 11:5

So far I think the answer is really: there is no idiomatic way of doing this (yet). – Incurious 24/3, 2012 at 23:7

Last response was a while ago -- is (str data.o.answer) idiomatic? – Sunsunbaked 27/12, 2015 at 6:50

You probably want aget:

(aget foo "list")

~~aget isn't variadic yet, but hopefully will be soon~~ it's variadic now. (aget data "o" "answer") would work

Romeo answered 22/3, 2012 at 16:26 Comment(2)

For the record, it's variadic now. (aget data "o" "answer") would work. – Trunk 28/2, 2013 at 15:20

FWIW, David would retract this answer these days, and suggest instead (goog.object/get foo "list"). See more in the answer by Jiacai Liu. – Manganite 29/1, 2018 at 14:22

Firstly, your proposed syntax for nested access does work:

ClojureScript:cljs.user> (def data 
    (JSON/parse "{\"list\": \"[1,2,3,4,5]\", \"blah\": \"vtha\", \"o\": {\"answer\": \"42\"}}"))
#<[object Object]>
ClojureScript:cljs.user> (.-answer (.-o data))
"42"

You can use the threading macros...

ClojureScript:cljs.user> (-> data (.-o) (.-answer))
"42"

Or .. notation

ClojureScript:cljs.user> (.. data -o -answer)
"42"

Stefanstefanac answered 22/3, 2012 at 11:7 Comment(3)

Does indeed work, still seems clunky to me, even with threading macros. – Incurious 24/3, 2012 at 23:8

the parens in the threading are redundant (-> data .-o .-answer) – Jock 14/4, 2014 at 16:0

@Jock these days I'd prefer to use (.. data -o -answer), as noted by @stephen-nelson - i'll update the answer – Stefanstefanac 14/4, 2014 at 20:47

If you're dealing with any amount of data, I'd convert the JSON into clojure data structures and then use the usual idioms:

(let [my-json (js* "{
                     list: [1,2,3,4,5],
                     blah: \"vtha\",
                     o: { answer: 42 }
                   }")
      converted (js->clj my-json)]

  (get-in converted ["list" 3]) ;; => 4
  (-> converted "o" "answer") ;;=> 42
)

(Note: don't use js* if you can help it; it's not idiomatic and might go away in future versions of ClojureScript.)

Whaleboat answered 22/3, 2012 at 14:39 Comment(3)

I'm finding that the data is converted into a vector rather than a hash, so actually more co,plicated to extract data from than the default js object – Incurious 24/3, 2012 at 23:3

Actually, the converted data seems to vary unpredictably ... obj map or vector – Incurious 24/3, 2012 at 23:4

This is like buying the whole grocery store just to get the oranges. A waste of resources. – Oesophagus 9/10, 2015 at 1:10

Clojurescript has a .. operator that is useful for chained javascript calls:

(.. data -o -answer) => data.o.answer => 42
(aget (.. data -list) 1) => data.list[1] => 2

You can use most list operators on arrays too, e.g.

(into [] (.. data -list)) ; vector [1 2 3 4]

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