SSCCE

result = Json.Decode.decodeValue (Json.Decode.dict Json.Decode.string) jsValue

when jsValue has been created in JavaScript using Object.create(null) and sent through flags or a port.

Attempting the decoding above results in a runtime exception with this error:

Uncaught TypeError: value.hasOwnProperty is not a function
    at _Json_runHelp (VM37 workspace:1461)
    at _Json_runHelp (VM37 workspace:1478)
    at Function.f (VM37 workspace:1400)
    at A2 (VM37 workspace:56)
    at $author$project$Main$init (VM37 workspace:10540)
    at VM37 workspace:8540
    at VM37 workspace:20
    at _Platform_initialize (VM37 workspace:1877)
    at VM37 workspace:3973
    at Object.init (VM37 workspace:20)

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Additional Details

• Elm: 0.19.1
• elm/json: 1.1.3
• Browser: Brave

Object.create(null) is sometimes used to create a JavaScript dictionary . The difference with {} is that {} contains fields from the Object prototype, where Object.create(null) has no prototype and no fields at all.

> ({}).toString
[Function: toString]
> Object.create(null).toString
undefined

This distinction, at least in JavaScript, can be important when you use this value as a set and only care about whether the field exists.

This distinction is irrelevant when decoding, but this runtime error makes it impossible to send values that in JS-land needed to be created this way without somehow cloning them beforehand.