Fix handling of partial / undefined props in React 18 TypeScript

Makefile

@@ -15,6 +15,7 @@ lint:
 	@$(BIN)/eslint lib
 	@$(BIN)/tsc --noEmit
 	@$(BIN)/tsc -p typings
+	@$(BIN)/tsc -p typings/tsconfig.react18.json
 
 # tsup emits cjs + esm + dts into build/cjs (and rewrites build/cjs/cjs.js to the
 # legacy module.exports === Draggable shape). webpack emits the UMD global bundle.

lib/Draggable.tsx

@@ -51,12 +51,20 @@ class Draggable extends React.Component<Partial<DraggableProps>, DraggableState>
 
   static displayName?: string = 'Draggable';
 
-  // The index-signature annotation is load-bearing: without it tsc infers the
-  // PropTypes.Requireable<...> types and emits `import PropTypes from 'prop-types'`
-  // into the generated public .d.ts, forcing consumers to install
-  // @types/prop-types (the v4.5.0 hand-written typings had no prop-types dep).
+  // Both the annotation and the `?` are load-bearing:
+  //  - The index-signature annotation stops tsc from inferring the
+  //    PropTypes.Requireable<...> types and emitting `import PropTypes from
+  //    'prop-types'` into the generated public .d.ts, which would force consumers
+  //    to install @types/prop-types (the v4.5.0 hand-written typings had none).
+  //  - The `?` keeps `propTypes` from being a *required* member of the public
+  //    type. React <= 18's JSX LibraryManagedAttributes only consults a
+  //    component's `propTypes` when it is required (`C extends {propTypes: ...}`);
+  //    when it does, this index-signature `propTypes` makes `defaultProps` stop
+  //    marking props optional, so consumers are forced to pass every prop.
+  //    Optional dodges that branch; React 19 ignores `propTypes` entirely. The
+  //    typings/tsconfig.react18.json check guards against a regression here.
   // Do not remove. See lib/DraggableCore.tsx for the same guard.
-  static propTypes: {[key: string]: unknown} = {
+  static propTypes?: {[key: string]: unknown} = {
     // Accepts all props <DraggableCore> accepts.
     ...DraggableCore.propTypes,
 

lib/DraggableCore.tsx

@@ -74,12 +74,20 @@ export default class DraggableCore extends React.Component<Partial<DraggableCore
 
   static displayName: string | undefined = 'DraggableCore';
 
-  // The index-signature annotation is load-bearing: without it tsc infers the
-  // PropTypes.Requireable<...> types and emits `import PropTypes from 'prop-types'`
-  // into the generated public .d.ts, forcing consumers to install
-  // @types/prop-types (the v4.5.0 hand-written typings had no prop-types dep).
+  // Both the annotation and the `?` are load-bearing:
+  //  - The index-signature annotation stops tsc from inferring the
+  //    PropTypes.Requireable<...> types and emitting `import PropTypes from
+  //    'prop-types'` into the generated public .d.ts, which would force consumers
+  //    to install @types/prop-types (the v4.5.0 hand-written typings had none).
+  //  - The `?` keeps `propTypes` from being a *required* member of the public
+  //    type. React <= 18's JSX LibraryManagedAttributes only consults a
+  //    component's `propTypes` when it is required (`C extends {propTypes: ...}`);
+  //    when it does, this index-signature `propTypes` makes `defaultProps` stop
+  //    marking props optional, so consumers are forced to pass every prop.
+  //    Optional dodges that branch; React 19 ignores `propTypes` entirely. The
+  //    typings/tsconfig.react18.json check guards against a regression here.
   // Do not remove. See lib/Draggable.tsx for the same guard.
-  static propTypes: {[key: string]: unknown} = {
+  static propTypes?: {[key: string]: unknown} = {
     /**
      * `allowAnyClick` allows dragging using any mouse button.
      * By default, we only accept the left button.

package.json

@@ -57,6 +57,7 @@
     "@types/node": "^25.0.3",
     "@types/prop-types": "^15.7.15",
     "@types/react": "^19.2.7",
+    "@types/react-18": "npm:@types/react@^18.3.27",
     "@types/react-dom": "^19.2.3",
     "@typescript-eslint/eslint-plugin": "^8.18.0",
     "@typescript-eslint/parser": "^8.18.0",

typings/tsconfig.react18.json

@@ -0,0 +1,23 @@
+{
+  "//": "Type-checks the public API against @types/react@18 as well as the default v19. React 18's JSX `LibraryManagedAttributes` consults a component's `propTypes`, unlike v19 which ignores it. A `propTypes` member leaking into the public types there makes `defaultProps` stop marking props optional, so React 18 consumers must pass every prop. v19 alone cannot catch that regression. Only `react` is redirected to the v18 types; `react-dom` stays on its installed v19 types (skipLibCheck ignores any v18/v19 mismatch inside its .d.ts). Aliasing react-dom too would pull in an `@types/react-dom-18` whose `@types/react@^18` peer cannot be satisfied under npm's strict resolution.",
+  "compilerOptions": {
+    "noEmit": true,
+    "jsx": "preserve",
+    "strict": true,
+    "target": "ES2019",
+    "lib": ["ES2019", "DOM", "DOM.Iterable"],
+    "moduleResolution": "node",
+    "esModuleInterop": true,
+    "baseUrl": ".",
+    "types": ["node"],
+    "skipLibCheck": true,
+    "paths": {
+      "react-draggable": ["../lib/cjs.ts"],
+      "react": ["../node_modules/@types/react-18"],
+      "react/jsx-runtime": ["../node_modules/@types/react-18/jsx-runtime"]
+    }
+  },
+  "files": [
+    "test.tsx"
+  ]
+}