From abdaf10d8805e3dab62b65dcc88b25090e660279 Mon Sep 17 00:00:00 2001
From: Evan You <yyx990803@gmail.com>
Date: Sat, 2 Nov 2019 10:40:08 -0400
Subject: [PATCH] chore: comments regarding Readonly usage

---
 packages/runtime-core/src/apiCreateComponent.ts | 2 ++
 packages/runtime-core/src/apiWatch.ts           | 3 +++
 2 files changed, 5 insertions(+)

diff --git a/packages/runtime-core/src/apiCreateComponent.ts b/packages/runtime-core/src/apiCreateComponent.ts
index c1503278ee..f4ab9bd379 100644
--- a/packages/runtime-core/src/apiCreateComponent.ts
+++ b/packages/runtime-core/src/apiCreateComponent.ts
@@ -68,6 +68,8 @@ export function createComponent<
 // overload 4: object format with object props declaration
 // see `ExtractPropTypes` in ./componentProps.ts
 export function createComponent<
+  // the Readonly constraint allows TS to treat the type of { required: true }
+  // as constant instead of boolean.
   PropsOptions extends Readonly<ComponentPropsOptions>,
   RawBindings,
   D,
diff --git a/packages/runtime-core/src/apiWatch.ts b/packages/runtime-core/src/apiWatch.ts
index 9f530661b2..8abe22ee93 100644
--- a/packages/runtime-core/src/apiWatch.ts
+++ b/packages/runtime-core/src/apiWatch.ts
@@ -68,6 +68,9 @@ export function watch<T>(
 ): StopHandle
 
 // overload #3: array of multiple sources + cb
+// Readonly constraint helps the callback to correctly infer value types based
+// on position in the source array. Otherwise the values will get a union type
+// of all possible value types.
 export function watch<T extends Readonly<WatcherSource<unknown>[]>>(
   sources: T,
   cb: WatchHandler<MapSources<T>>,
-- 
2.47.3