How to return js expressions from a macro?

[ranfdev]

The code

(prn #js [1 2])

correctly compiles to

cherry_core.prn.call(null, [1, 2]);

But now let's say that I want to refactor said code with a macro, by doing

;; file macros.cljs
(ns macros)
(defmacro append-two-then-as-js [x]
    (quote #js [~x 2])
    
;; file main.js 
(ns main
  (:require-macros
   [macros :refer [append-two-then-as-js]]))
(prn (append-two-then-as-js 1))

The resulting code is

cherry_core.prn.call(null, [object Object]);

Problem:
The #js is a reader macro that gets expanded during the evaluation of the macro append-two-then-as-js. But that is arguably useless, because now the compiled code contains [object Object].

Question:
How can we write macros returning #js expressions? I can't find a way to write a macro returning #js [1 2] as is

[borkdude]

You can't do that. Just don't use #js [1 2] in a macro, since the reader is executed at macro expansion time. If you want to return code that constructs a JS array at runtime, just write something like:

`(array ~x 2)

[ranfdev]

Thanks, I didn't know about that possibility.

Do you think it would be possible to write a generic (as-js form) macro, capable of doing the same thing as #js, by manually checking the type of form and calling the needed function, as (array x) for vectors...?

Would it make sense to have this theoretical (as-js form) macro, and implement #js in terms of it, so that #js [1 2] gets translated to (as-js [1 2])?

[borkdude]

The thing you're looking for is probably clj->js

[ranfdev]

Mmm... Not really, because I can't call clj->js for every object or array literal, for performance reasons. But I will use a macro to call (array form), (js-obj form) and (clj->js) as needed.

Thanks!

[ranfdev]

(js-obj) and (array) though are translated in runtime function calls, while #js {} and #js [] don't.
I really can't find a way to return js literals from a macro

[borkdude]

Actually after digging in some more, if you would return from the macro:

(vary-meta {} assoc :cherry.compiler/js true)

on the returned map, I think cherry might already translate it to a JS object at compile time.

[ranfdev]

Indeed, that works, and that's exactly what I was looking for. Thanks

[borkdude]

@ranfdev In the latest cherry version the key changed to :squint.compiler-common/js. But instead I think it's better to just optimize js-obj and array to emit literals when possible. I don't think CLJS has a way to emit JS objects from macros either other than js-obj and array.

[borkdude]

Here's the issue: #141

[ranfdev]

Thanks I'll look into it