From b1c35738204b5b1a5d325a16e70cdbf25b523976 Mon Sep 17 00:00:00 2001
From: John Chadwick <john@jchw.io>
Date: Thu, 23 Jan 2020 07:12:22 -0800
Subject: [PATCH] fix(toPromise): correct toPromise return type (#5072)

* fix(toPromise): correct toPromise return type

* fix(toPromise): add test for toPromise type

BREAKING CHANGE: toPromise return type now returns `T | undefined` in TypeScript, which is correct, but may break builds.
---
 spec-dtslint/Observable-spec.ts |  4 ++++
 src/internal/Observable.ts      | 12 ++++++------
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/spec-dtslint/Observable-spec.ts b/spec-dtslint/Observable-spec.ts
index ee7c1b1053..3033f6f1bf 100644
--- a/spec-dtslint/Observable-spec.ts
+++ b/spec-dtslint/Observable-spec.ts
@@ -126,4 +126,8 @@ describe('pipe', () => {
     const customOperator = () => <T>(a: Observable<T>) => a;
     const o = of('foo').pipe(customOperator()); // $ExpectType Observable<string>
   });
+
+  it('should have proper return type for toPromise', () => {
+    const o = of('foo').toPromise(); // $ExpectType Promise<string | undefined>
+  });
 });
diff --git a/src/internal/Observable.ts b/src/internal/Observable.ts
index fbea2874ab..02c96a79e7 100644
--- a/src/internal/Observable.ts
+++ b/src/internal/Observable.ts
@@ -347,9 +347,9 @@ export class Observable<T> implements Subscribable<T> {
   }
 
   /* tslint:disable:max-line-length */
-  toPromise<T>(this: Observable<T>): Promise<T>;
-  toPromise<T>(this: Observable<T>, PromiseCtor: typeof Promise): Promise<T>;
-  toPromise<T>(this: Observable<T>, PromiseCtor: PromiseConstructorLike): Promise<T>;
+  toPromise<T>(this: Observable<T>): Promise<T | undefined>;
+  toPromise<T>(this: Observable<T>, PromiseCtor: typeof Promise): Promise<T | undefined>;
+  toPromise<T>(this: Observable<T>, PromiseCtor: PromiseConstructorLike): Promise<T | undefined>;
   /* tslint:enable:max-line-length */
 
   /**
@@ -362,13 +362,13 @@ export class Observable<T> implements Subscribable<T> {
    * @return A Promise that resolves with the last value emit, or
    * rejects on an error.
    */
-  toPromise(promiseCtor?: PromiseConstructorLike): Promise<T> {
+  toPromise(promiseCtor?: PromiseConstructorLike): Promise<T | undefined> {
     promiseCtor = getPromiseCtor(promiseCtor);
 
     return new promiseCtor((resolve, reject) => {
-      let value: any;
+      let value: T | undefined;
       this.subscribe((x: T) => value = x, (err: any) => reject(err), () => resolve(value));
-    }) as Promise<T>;
+    }) as Promise<T | undefined>;
   }
 }