Program to reproduce

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE PackageImports #-}

module Main where

import qualified "aeson" Data.Aeson as AE
import           "ghcjs-base" GHCJS.Types ( jsval )
import           "ghcjs-base" GHCJS.Marshal ( fromJSValUnchecked )
import           "ghcjs-base" GHCJS.Foreign ( jsTrue )
import qualified "ghcjs-base" JavaScript.Object as Obj

main :: IO ()
main = do
  putStrLn "Creating object"
  obj <- Obj.create

  Obj.setProp "foo" jsTrue obj

  putStrLn "Object -> JSVal"
  let jsv = jsval obj

  putStrLn "Going to Aeson"
  x <- (fromJSValUnchecked jsv) :: IO AE.Value -- <-- Error here

  putStrLn $ seq x "Evaluated x"

Output

Creating object

lib.js:4616 Object -> JSVal

lib.js:4616 Going to Aeson

rts.js:7062 uncaught exception in Haskell main thread: TypeError: Cannot read property 't' of undefined
rts.js:7062 TypeError: Cannot read property 't' of undefined
    at h$e (rts.js:18673)
    at h$$Bz (out.js:28148)
    at h$runThreadSlice (rts.js:8012)
    at h$runThreadSliceCatch (rts.js:7968)
    at h$mainLoop (rts.js:7963)
    at rts.js:2460
    at runIfPresent (rts.js:2478)
    at onGlobalMessage (rts.js:2512)
rts.js:8313 Uncaught h$ThreadAbortedError {code: 0}
h$exitProcess @ rts.js:8313
h$doneMain @ rts.js:8298
h$doneMain_e @ rts.js:12230
h$runThreadSliceCatch @ rts.js:7975
h$mainLoop @ rts.js:7963
(anonymous) @ rts.js:2460
runIfPresent @ rts.js:2478
onGlobalMessage @ rts.js:2512

ghcjs version: The Glorious Glasgow Haskell Compilation System for JavaScript, version 0.2.1 (GHC 8.0.2) ghcjs-0.2.020161101

Package versions:

    aeson-0.11.2.0
    array-0.5.1.1
    attoparsec-0.13.0.2
    base-4.9.0.0
    binary-0.8.3.0
    bytestring-0.10.8.1
    containers-0.5.7.1
    deepseq-1.4.2.0
    directory-1.2.6.2
    dlist-0.7.1.2
    fail-4.9.0.0
    filepath-1.4.1.0
    ghc-8.0.1
    ghc-boot-8.0.1
    ghc-boot-th-8.0.1
    ghc-prim-0.5.0.0
    ghci-8.0.1
    ghcjs-base-0.2.0.0
    ghcjs-dom-0.7.0.4
    ghcjs-dom-jsffi-0.7.0.4
    ghcjs-ffiqq-0.1.0.0
    ghcjs-prim-0.1.0.0
    ghcjs-th-0.1.0.0
    hashable-1.2.4.0
    integer-gmp-1.0.0.1
    mtl-2.2.2
    pretty-1.1.3.3
    primitive-0.6.1.0
    process-1.4.2.0
    rts-1.0
    scientific-0.3.4.7
    split-0.2.3.1
    syb-0.6
    tagged-0.8.5
    template-haskell-2.11.0.0
    text-1.2.2.1
    time-1.6.0.1
    transformers-0.5.2.0
    transformers-compat-0.5.1.4
    unix-2.7.2.0
    unordered-containers-0.2.7.0
    vector-0.11.0.0

I think this has been fixed (probably by https://github.com/ghcjs/shims/pull/41). I get:

$node test.jsexe/all.js
Creating object
Object -> JSVal
Going to Aeson
Evaluated x
$ ghcjs-pkg list
/Users/hamish/.ghcjs/x86_64-darwin-0.2.1-8.0.2/ghcjs/package.conf.d
    Cabal-1.24.0.0
    aeson-0.11.2.0
    array-0.5.1.1
    async-2.1.0
    attoparsec-0.13.0.2
    base-4.9.0.0
    binary-0.8.3.0
    bytestring-0.10.8.1
    case-insensitive-1.2.0.6
    containers-0.5.7.1
    deepseq-1.4.2.0
    directory-1.2.6.2
    dlist-0.7.1.2
    extensible-exceptions-0.1.1.4
    fail-4.9.0.0
    filepath-1.4.1.0
    ghc-8.0.1
    ghc-boot-8.0.1
    ghc-boot-th-8.0.1
    ghc-prim-0.5.0.0
    ghci-8.0.1
    ghcjs-base-0.2.0.0
    ghcjs-prim-0.1.0.0
    ghcjs-th-0.1.0.0
    hashable-1.2.4.0
    integer-gmp-1.0.0.1
    mtl-2.2.2
    parallel-3.2.1.0
    pretty-1.1.3.3
    primitive-0.6.1.0
    process-1.4.2.0
    rts-1.0
    scientific-0.3.4.7
    stm-2.4.4.1
    syb-0.6
    tagged-0.8.4
    template-haskell-2.11.0.0
    text-1.2.2.1
    time-1.6.0.1
    transformers-0.5.2.0
    unix-2.7.2.0
    unordered-containers-0.2.7.0
    vector-0.11.0.0

ghcjs / ghcjs-base

FromJSVal Aeson instance broken w.r.t. objects. #88

Program to reproduce

Output