fix(pairwise): make it recursion-proof (#4743)

Author: thorn0

spec/operators/pairwise-spec.ts

@@ -1,5 +1,7 @@
 import{hot,cold,expectObservable,expectSubscriptions}from'../helpers/marble-testing';
-import{pairwise}from'rxjs/operators';
+import{pairwise,take}from'rxjs/operators';
+import{Subject}from'rxjs';
+import{expect}from'chai';
 
 declarefunctionasDiagram(arg: string): Function;
 
@@ -103,4 +105,25 @@ describe('pairwise operator', () => {
 expectObservable(source).toBe(expected);
 expectSubscriptions(e1.subscriptions).toBe(e1subs);
 });
+
+it('should be recursively re-enterable',()=>{
+constresults=newArray<[string,string]>();
+
+constsubject=newSubject<string>();
+
+subject
+.pipe(
+pairwise(),
+take(3)
+)
+.subscribe(pair=>{
+results.push(pair);
+subject.next('c');
+});
+
+subject.next('a');
+subject.next('b');
+
+expect(results).to.deep.equal([['a','b'],['b','c'],['c','c']]);
+});
 });

src/internal/operators/pairwise.ts

@@ -70,12 +70,18 @@ class PairwiseSubscriber<T> extends Subscriber<T> {
 }
 
 _next(value: T): void{
+letpair: [T,T]|undefined;
+
 if(this.hasPrev){
-this.destination.next([this.prev,value]);
+pair=[this.prev,value];
 }else{
 this.hasPrev=true;
 }
 
 this.prev=value;
+
+if(pair){
+this.destination.next(pair);
+}
 }
 }