Fixed BTreeMap deserialization from an empty JSON object

crates/vertigo/src/driver_module/js_value/serialize.rs

@@ -211,21 +211,41 @@ impl<K: JsJsonSerialize + JsJsonDeserialize + Ord, T: JsJsonSerialize + JsJsonDe
     JsJsonDeserialize for BTreeMap<K, T>
 {
     fn from_json(context: JsJsonContext, json: JsJson) -> Result<Self, JsJsonContext> {
-        let JsJson::List(list) = json else {
-            let message = ["list expected, received ", json.typename()].concat();
-            return Err(context.add(message));
-        };
-
         let mut result = BTreeMap::new();
 
-        for (index, item) in list.into_iter().enumerate() {
-            let item = from_json::<MapItem<K, T>>(item)
-                .map_err(|err| context.add(format!("index={index} error={err}")))?;
-
-            let exist = result.insert(item.key, item.value);
-
-            if exist.is_some() {
-                return Err(context.add("Duplicate key"));
+        match json {
+            // Native vertigo form: a list of `{k, v}` items.
+            JsJson::List(list) => {
+                for (index, item) in list.into_iter().enumerate() {
+                    let item = from_json::<MapItem<K, T>>(item)
+                        .map_err(|err| context.add(format!("index={index} error={err}")))?;
+
+                    if result.insert(item.key, item.value).is_some() {
+                        return Err(context.add("Duplicate key"));
+                    }
+                }
+            }
+            // Interop form (e.g. produced by serde): a plain object `{"key": value}`.
+            // Synthesize a `{k, v}` MapItem per entry and delegate. Non-String keys
+            // surface a graceful error because the key arrives as a JsJson::String.
+            JsJson::Object(obj) => {
+                for (key, value) in obj {
+                    let synthetic = JsJson::Object(BTreeMap::from([
+                        ("k".to_string(), JsJson::String(key.clone())),
+                        ("v".to_string(), value),
+                    ]));
+
+                    let item = from_json::<MapItem<K, T>>(synthetic)
+                        .map_err(|err| context.add(format!("key='{key}' error={err}")))?;
+
+                    if result.insert(item.key, item.value).is_some() {
+                        return Err(context.add("Duplicate key"));
+                    }
+                }
+            }
+            other => {
+                let message = ["list or object expected, received ", other.typename()].concat();
+                return Err(context.add(message));
             }
         }
 

crates/vertigo/src/driver_module/js_value/tests.rs

@@ -1,7 +1,7 @@
-use std::collections::BTreeMap;
+use std::{collections::BTreeMap, error::Error};
 
 use super::{
-    MemoryBlock, MemoryBlockWrite,
+    MemoryBlock, MemoryBlockWrite, from_json,
     js_json_struct::{JsJson, JsJsonNumber},
 };
 
@@ -67,6 +67,71 @@ fn json_json_list() {
     assert_eq!(data1, data2);
 }
 
+#[test]
+fn btreemap_from_empty_object() -> Result<(), Box<dyn Error>> {
+    // An empty JSON object `{}` is a valid representation of an empty map.
+    let json = JsJson::Object(BTreeMap::new());
+    let result = from_json::<BTreeMap<String, u32>>(json)?;
+    assert!(result.is_empty());
+    Ok(())
+}
+
+#[test]
+fn btreemap_from_empty_list() -> Result<(), Box<dyn Error>> {
+    // An empty list `[]` still deserializes to an empty map.
+    let json = JsJson::List(vec![]);
+    let result = from_json::<BTreeMap<String, u32>>(json)?;
+    assert!(result.is_empty());
+    Ok(())
+}
+
+#[test]
+fn btreemap_from_non_empty_object_string_keys() -> Result<(), Box<dyn Error>> {
+    // serde serializes BTreeMap<String, V> as a plain object `{"key": value}`,
+    // so the object form must deserialize into a map for String keys.
+    let json = JsJson::Object(BTreeMap::from([
+        ("foo".to_string(), JsJson::Number(JsJsonNumber(1.0))),
+        ("bar".to_string(), JsJson::Number(JsJsonNumber(2.0))),
+    ]));
+    let result = from_json::<BTreeMap<String, u32>>(json)?;
+    assert_eq!(result.len(), 2);
+    assert_eq!(result.get("foo"), Some(&1));
+    assert_eq!(result.get("bar"), Some(&2));
+    Ok(())
+}
+
+#[test]
+fn btreemap_from_object_non_string_keys_errors() {
+    // Object form only makes sense for String keys; numeric keys arrive as
+    // strings and must fail gracefully.
+    let json = JsJson::Object(BTreeMap::from([(
+        "1".to_string(),
+        JsJson::Number(JsJsonNumber(10.0)),
+    )]));
+    assert!(from_json::<BTreeMap<u32, u32>>(json).is_err());
+}
+
+#[test]
+fn btreemap_from_list_of_items() -> Result<(), Box<dyn Error>> {
+    // A populated list of `{k, v}` items round-trips into a map.
+    let json = JsJson::List(vec![
+        JsJson::Object(BTreeMap::from([
+            ("k".to_string(), JsJson::String("aaa".into())),
+            ("v".to_string(), JsJson::Number(JsJsonNumber(1.0))),
+        ])),
+        JsJson::Object(BTreeMap::from([
+            ("k".to_string(), JsJson::String("bbb".into())),
+            ("v".to_string(), JsJson::Number(JsJsonNumber(2.0))),
+        ])),
+    ]);
+
+    let result = from_json::<BTreeMap<String, u32>>(json)?;
+    assert_eq!(result.len(), 2);
+    assert_eq!(result.get("aaa"), Some(&1));
+    assert_eq!(result.get("bbb"), Some(&2));
+    Ok(())
+}
+
 #[test]
 fn json_json_vec() {
     let data1 = JsJson::Vec(vec![1, 2, 3, 4, 5]);

docs/CHANGELOG.md

@@ -14,6 +14,7 @@
 * Partially fixed SVG tags rendering, for full fix trace [#539]
 * `TwClass` in dynamic attribute (`dyn:tw={my_tw_class}`)
 * `dom!` now correctly handles block of statements with attribute name inferred from variable name in the last statement
+* `BTreeMap` deserialization now accepts objects `{...}` as well.
 
 ## 0.11.4 - 2026-04-18