Add undefined to return type of PropertyValues.get() (#3710)

.changeset/tiny-baboons-invent.md

@@ -0,0 +1,5 @@
+---
+'@lit/reactive-element': patch
+---
+
+Add `undefined` to the return type of PropertyValues.get()

packages/reactive-element/src/reactive-element.ts

@@ -288,7 +288,7 @@ export type PropertyValues<T = any> = T extends object
 // This type must be exported such that JavaScript generated by the Google
 // Closure Compiler can import a type reference.
 export interface PropertyValueMap<T> extends Map<PropertyKey, unknown> {
-  get<K extends keyof T>(k: K): T[K];
+  get<K extends keyof T>(k: K): T[K] | undefined;
   set<K extends keyof T>(key: K, value: T[K]): this;
   has<K extends keyof T>(k: K): boolean;
   delete<K extends keyof T>(k: K): boolean;

packages/reactive-element/src/test/reactive-element_test.ts

@@ -3233,7 +3233,7 @@ suite('ReactiveElement', () => {
           changedProperties.set('foo', 'hi');
 
           // This should type-check without a cast:
-          const x: number = changedProperties.get('foo');
+          const x: number | undefined = changedProperties.get('foo');
           changedProperties.set('foo', 2);
 
           // This should type-check without a cast:
@@ -3295,7 +3295,7 @@ suite('ReactiveElement', () => {
     class A extends ReactiveElement {
       foo!: number;
       override update(changedProperties: PropertyValues<A>) {
-        const n: number = changedProperties.get('foo');
+        const n: number | undefined = changedProperties.get('foo');
         if (n) {
           //Suppress no-unused-vars warnings
         }
@@ -3304,7 +3304,7 @@ suite('ReactiveElement', () => {
     class B extends A {
       bar!: string;
       override update(changedProperties: PropertyValues<B>) {
-        const s: string = changedProperties.get('bar');
+        const s: string | undefined = changedProperties.get('bar');
         if (s) {
           //Suppress no-unused-vars warnings
         }