feat: add typed array support to Json serialization (#182)

Add support for serializing F# typed arrays (Int32Array, Int64Array, etc.)
to JSON by converting them to Python lists.

Supported array types:
- Generic arrays: FSharpArray, GenericArray
- Typed arrays: Int8Array, Int16Array, Int32Array, Int64Array,
  UInt8Array, UInt16Array, UInt32Array, UInt64Array,
  Float32Array, Float64Array

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: dbrattli

release-please-config.json

@@ -3,7 +3,7 @@
   "packages": {
     ".": {
       "release-type": "simple",
-      "release-as": "5.0.0-alpha.21.1",
+      "release-as": "5.0.0-alpha.21.2",
       "include-component-in-tag": false,
       "include-v-in-tag": true,
       "changelog-path": "CHANGELOG.md"

src/stdlib/Json.fs

@@ -86,9 +86,13 @@ let private getCases (o: obj) : string array = nativeOnly
 [<Emit("(_ for _ in ()).throw(TypeError(f'Object of type {type($0).__name__} is not JSON serializable'))")>]
 let private raiseTypeError (o: obj) : obj = nativeOnly
 
+[<Emit("list($0)")>]
+let private toList (o: obj) : obj = nativeOnly
+
 /// Default function for JSON serialization of Fable types.
 /// Handles Int8, Int16, Int32, Int64, UInt8, UInt16, UInt32, UInt64 → int
 /// Handles Float32, Float64 → float
+/// Handles typed arrays (Int32Array, Int64Array, Float64Array, etc.) → list
 /// Handles Union types (tag, fields, cases) → [caseName, ...fields] or just caseName
 /// Handles Record types (__slots__) → dict of slot names to values
 let fableDefault (o: obj) : obj =
@@ -105,6 +109,20 @@ let fableDefault (o: obj) : obj =
     | "UInt64" -> toInt o
     | "Float32"
     | "Float64" -> toFloat o
+    // Generic arrays
+    | "FSharpArray"
+    | "GenericArray"
+    // Typed arrays
+    | "Int8Array"
+    | "Int16Array"
+    | "Int32Array"
+    | "Int64Array"
+    | "UInt8Array"
+    | "UInt16Array"
+    | "UInt32Array"
+    | "UInt64Array"
+    | "Float32Array"
+    | "Float64Array" -> toList o
     | _ ->
         if hasattr o "tag" && hasattr o "fields" then
             let cases = getCases o

test/TestJson.fs

@@ -94,3 +94,27 @@ let ``test Json.dumps with union case with multiple fields works`` () =
     let union = CaseC("hello", 123)
     let result = Json.dumps union
     result |> equal """["CaseC", "hello", 123]"""
+
+[<Fact>]
+let ``test Json.dumps with F# array works`` () =
+    let values = [| 1; 2; 3 |]
+    let result = Json.dumps values
+    result |> equal "[1, 2, 3]"
+
+[<Fact>]
+let ``test Json.dumps with F# array of strings works`` () =
+    let values = [| "a"; "b"; "c" |]
+    let result = Json.dumps values
+    result |> equal """["a", "b", "c"]"""
+
+[<Fact>]
+let ``test Json.dumps with nested F# array works`` () =
+    let obj = {| Items = [| 1; 2; 3 |] |}
+    let result = Json.dumps obj
+    result |> equal """{"Items": [1, 2, 3]}"""
+
+[<Fact>]
+let ``test Json.dumps with empty F# array works`` () =
+    let values: int array = [||]
+    let result = Json.dumps values
+    result |> equal "[]"