From 7db30394a96edd49b1f7281c2b105418934e717c Mon Sep 17 00:00:00 2001
From: Grant Nelson <grant.nelson@workiva.com>
Date: Tue, 30 Apr 2024 10:04:34 -0600
Subject: [PATCH] Fixing internalization of null slice and array fields

---
 compiler/prelude/jsmapping.js |  6 ++++++
 tests/js_test.go              | 39 +++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/compiler/prelude/jsmapping.js b/compiler/prelude/jsmapping.js
index b22454bc3..f5317d626 100644
--- a/compiler/prelude/jsmapping.js
+++ b/compiler/prelude/jsmapping.js
@@ -236,6 +236,9 @@ var $internalize = (v, t, recv, seen, makeWrapper) => {
         case $kindFloat64:
             return parseFloat(v);
         case $kindArray:
+            if (v === null || v === undefined) {
+                $throwRuntimeError("cannot internalize "+v+" as a "+t.string);
+            }
             if (v.length !== t.len) {
                 $throwRuntimeError("got array with wrong size from JavaScript native");
             }
@@ -331,6 +334,9 @@ var $internalize = (v, t, recv, seen, makeWrapper) => {
                 return $internalize(v, t.elem, makeWrapper);
             }
         case $kindSlice:
+            if (v == null) {
+                return t.zero();
+            }
             return new t($mapArray(v, e => { return $internalize(e, t.elem, makeWrapper); }));
         case $kindString:
             v = String(v);
diff --git a/tests/js_test.go b/tests/js_test.go
index 2ce43865f..6f6eaa542 100644
--- a/tests/js_test.go
+++ b/tests/js_test.go
@@ -829,6 +829,45 @@ func TestExternalize(t *testing.T) {
 	}
 }
 
+func TestInternalizeSlice(t *testing.T) {
+	tests := []struct {
+		name string
+		init []int
+		want string
+	}{
+		{
+			name: `nil slice`,
+			init: []int(nil),
+			want: `[]int(nil)`,
+		},
+		{
+			name: `empty slice`,
+			init: []int{},
+			want: `[]int{}`,
+		},
+		{
+			name: `non-empty slice`,
+			init: []int{42, 53, 64},
+			want: `[]int{42, 53, 64}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			b := struct {
+				*js.Object
+				V []int `js:"V"` // V is externalized
+			}{Object: js.Global.Get("Object").New()}
+			b.V = tt.init
+
+			result := fmt.Sprintf(`%#v`, b.V) // internalize b.V
+			if result != tt.want {
+				t.Errorf(`Unexpected result %q != %q`, result, tt.want)
+			}
+		})
+	}
+}
+
 func TestInternalizeExternalizeNull(t *testing.T) {
 	type S struct {
 		*js.Object