Generated JS missing null check when pattern matching JSON.Array and JSON.Object

[jagguji]

Bug: Switch pattern matching on JSON.Array and JSON.Object generates incorrect JS when Null case is omitted

When pattern matching on JSON values with both Array and Object cases, the generated JavaScript incorrectly omits the null check, causing runtime failures
when the value is null.

Reproducible Example

This ReScript code generates incorrect JavaScript (missing null check):

  let getSchedule = (dict, key) => {
    dict
    ->Dict.get(key)
    ->Option.mapOr(None, schedule => {
      switch schedule {
      | JSON.Array(array) =>
        Some(
          array->Array.map(finalJson => {
            let finalDict = finalJson->getDictFromJsonObject
            mappingScheduleWithDict(finalDict)
          }),
        )
      | JSON.Object(dict) => Some([mappingScheduleWithDict(dict)])
      | _ => None
      }
    })
  }

Incorrect Output (Bug): this is not handling the null case which should go to the None

  function getSchedule(dict, key) {
    return Stdlib_Option.mapOr(dict[key], undefined, schedule => {
      if (Array.isArray(schedule)) {  // <-- Crashes if schedule is null!
        return schedule.map(finalJson => mappingScheduleWithDict(LogicUtils.getDictFromJsonObject(finalJson)));
      }
      switch (typeof schedule) {
        case "object" :
          return [mappingScheduleWithDict(schedule)];
        default:
          return;
      }
    });
  }

Workaround - Adding explicit Null case generates correct code:

  | Null | _ => None

Correct Output (with Null case):

  if (schedule === null) {
    return;
  }

Additional Evidence - Only Object case generates correct guards:

  switch schedule {
  | JSON.Object(dict) => Some([mappingScheduleWithDict(dict)])
  | _ => None
  }

Correctly generates:

  if (typeof schedule === "object" && schedule !== null && !Array.isArray(schedule))

Expected Behavior

When both Array and Object cases are present without explicit Null, the compiler should still emit the null check before Array.isArray() to prevent runtime
errors, consistent with the single Object case behavior.

Environment

• ReScript version: 12.0.0
• OS: macOS / Linux

[jagguji]

@zth

[jagguji]

switch (item: JSON.t) {
| String(_str) => stringArray->Array.push(item)
| Object(_obj) => {
let flattenedSubObj = flatten(item, addIndicatorForObject)
flattenedSubObj
->Dict.toArray
->Array.forEach(
subEntry => {
let (subKey, subValue) = subEntry
Dict.set(newDict, ${key}[${index->Int.toString}].${subKey}, subValue)
},
)
}

          | _ => arrayArray->Array.push(item)
          }

even in this case the generated js is
switch (typeof item) {
case "string" :
stringArray.push(item);
return;
case "object" :
let flattenedSubObj = flatten(item, addIndicatorForObject);
Object.entries(flattenedSubObj).forEach(subEntry => {
newDict[key + [ + index.toString() + ]. + subEntry[0]] = subEntry[1];
});
return;
default:
arrayArray.push(item);
return;
}
and even the array is going to the object case only

[jagguji]

@cristianoc can u confirm if this will handle the array going to object case as well ?

switch jsonArray { | Object() => "Object" | String() => "String" | _ => "default" }
switch jsonArray { | Object() => "Object" | Number() => "Number" | _ => "default" }
switch jsonArray { | Object() => "Object" | Bool(_) => "Boolean" | _ => "default" }

[cristianoc]

@cristianoc can u confirm if this will handle the array going to object case as well ?

switch jsonArray { | Object() => "Object" | String() => "String" | _ => "default" } switch jsonArray { | Object() => "Object" | Number() => "Number" | _ => "default" } switch jsonArray { | Object() => "Object" | Bool(_) => "Boolean" | _ => "default" }

I think these are handled well (but I don't know the entire context of the file). If you find one more reproduction case that is still wrong on master, please do report it.

[jagguji]

are we going to release another version of rescript 12 with the above fix ?
if yes then can u tell me when are we planning ? @cristianoc

[jagguji]

switch JSON.Encode.array([]) { | Object() => "Object" | String() => "String" | _ => "default" }
actually this was also falling under the object case instead of default, this issue is there in rescript 12

[nojaf]

We may attempt to publish a new release by the end of the week, but I cannot make any promises.

That being said, it is worth reminding you that all maintainers are volunteers and there is no large company behind this project. Therefore, no one is entitled to guaranteed support or releases. As an open source project, at minimum you can inspect the source code. If that is not satisfactory, I do not believe the license prevents you from publishing a fork to npm to unblock yourself, or from distributing the compiled artifact by other means. Overall, there should be ways to work around your current problem.

[jagguji]

@cristianoc the array case is also not handled it seems,
switch JSON.Encode.array([]) { | Object() => "Object" | String() => "String" | _ => "default" } this case will match with the object

[jagguji]

can u pls reopen this issue ?