fix(scan): scan operator now accepts undefined itself as a valid se…

…ed value

Array#reduce supports `undefined` as a valid seed value, so it seems
natural that we would too for scan

```js
of(1, 2, 3).scan((acc, x) => acc + ' ' + x, undefined);
// "undefined 1"
// "undefined 1 2"
// "undefined 1 2 3"
```

fixes ReactiveX#2047

spec/operators/scan-spec.ts

@@ -43,6 +43,26 @@ describe('Observable.prototype.scan', () => {
     expectSubscriptions(e1.subscriptions).toBe(e1subs);
   });
 
+  it('should scan with a seed of undefined', () => {
+    const e1 = hot('--a--^--b--c--d--e--f--g--|');
+    const e1subs =      '^                    !';
+    const expected =    '---u--v--w--x--y--z--|';
+
+    const values = {
+      u: 'undefined b',
+      v: 'undefined b c',
+      w: 'undefined b c d',
+      x: 'undefined b c d e',
+      y: 'undefined b c d e f',
+      z: 'undefined b c d e f g'
+    };
+
+    const source = e1.scan((acc: any, x: string) => acc + ' ' + x, undefined);
+
+    expectObservable(source).toBe(expected, values);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
   it('should scan without seed', () => {
     const e1 = hot('--a--^--b--c--d--|');
     const e1subs =      '^           !';

src/operator/scan.ts

@@ -45,15 +45,24 @@ export function scan<T>(this: Observable<T>, accumulator: (acc: T[], value: T, i
 export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed?: R): Observable<R>;
 /* tslint:disable:max-line-length */
 export function scan<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed?: T | R): Observable<R> {
-  return this.lift(new ScanOperator(accumulator, seed));
+  let hasSeed = false;
+  // providing a seed of `undefined` *should* be valid and trigger
+  // hasSeed! so don't use `seed !== undefined` checks!
+  // For this reason, we have to check it here at the original call site
+  // otherwise inside Operator/Subscriber we won't know if `undefined`
+  // means they didn't provide anything or if they literally provided `undefined`
+  if (arguments.length >= 2) {
+    hasSeed = true;
+  }
+
+  return this.lift(new ScanOperator(accumulator, seed, hasSeed));
 }
 
 class ScanOperator<T, R> implements Operator<T, R> {
-  constructor(private accumulator: (acc: R, value: T, index: number) => R, private seed?: T | R) {
-  }
+  constructor(private accumulator: (acc: R, value: T, index: number) => R, private seed?: T | R, private hasSeed: boolean = false) {}
 
   call(subscriber: Subscriber<R>, source: any): any {
-    return source._subscribe(new ScanSubscriber(subscriber, this.accumulator, this.seed));
+    return source._subscribe(new ScanSubscriber(subscriber, this.accumulator, this.seed, this.hasSeed));
   }
 }
 
@@ -64,26 +73,22 @@ class ScanOperator<T, R> implements Operator<T, R> {
  */
 class ScanSubscriber<T, R> extends Subscriber<T> {
   private index: number = 0;
-  private accumulatorSet: boolean = false;
-  private _seed: T | R;
 
   get seed(): T | R {
     return this._seed;
   }
 
   set seed(value: T | R) {
-    this.accumulatorSet = true;
+    this.hasSeed = true;
     this._seed = value;
   }
 
-  constructor(destination: Subscriber<R>, private accumulator: (acc: R, value: T, index: number) => R, seed?: T | R) {
+  constructor(destination: Subscriber<R>, private accumulator: (acc: R, value: T, index: number) => R, private _seed: T | R, private hasSeed: boolean) {
     super(destination);
-    this.seed = seed;
-    this.accumulatorSet = typeof seed !== 'undefined';
   }
 
   protected _next(value: T): void {
-    if (!this.accumulatorSet) {
+    if (!this.hasSeed) {
       this.seed = value;
       this.destination.next(value);
     } else {