Merge d511b52 into 8a1c84c

__tests__/base.js

@@ -122,6 +122,25 @@ function runBaseTest(name, useProxies, freeze, useListener) {
             expect(nextState.anObject.nested).toBe(undefined)
         })
 
+        // Found by: https://github.com/mweststrate/immer/pull/267
+        it("can delete props added in the producer", () => {
+            const nextState = produce(
+                baseState,
+                s => {
+                    s.anObject.test = true
+                    delete s.anObject.test
+                },
+                listener
+            )
+            if (useProxies) {
+                expect(nextState).not.toBe(baseState)
+                expect(nextState).toEqual(baseState)
+            } else {
+                // The copy is avoided in ES5.
+                expect(nextState).toBe(baseState)
+            }
+        })
+
         it("ignores single non-modification", () => {
             const nextState = produce(
                 baseState,

src/proxy.js

@@ -107,8 +107,11 @@ function set(state, prop, value) {
 }
 
 function deleteProperty(state, prop) {
-    state.assigned[prop] = false
-    markChanged(state)
+    // The `undefined` check is a fast path for pre-existing keys.
+    if (state.base[prop] !== undefined || prop in state.base) {
+        state.assigned[prop] = false
+        markChanged(state)
+    }
     delete state.copy[prop]
     return true
 }
@@ -117,8 +120,8 @@ function getOwnPropertyDescriptor(state, prop) {
     const owner = state.modified
         ? state.copy
         : has(state.proxies, prop)
-            ? state.proxies
-            : state.base
+        ? state.proxies
+        : state.base
     const descriptor = Reflect.getOwnPropertyDescriptor(owner, prop)
     if (descriptor && !(Array.isArray(owner) && prop === "length"))
         descriptor.configurable = true