fix(throttleTime): emit single value with trailing enabled (#4564)

* test(throttleTime): test single value with trailing enabled
Test if throttleTime emits when only a single value is emitted from the
source with leading enabled/disabled and trailing enabled.
* fix(throttleTime): fix single value with leading disabled, trailing enabled
Emit single value with leading disabled, trailing enabled.
Closes#2859 and #4491.
* chore: improve test descriptions
And remove an asDiagram call made with the same argument.
Signed-off-by: Matthias Kunnen <matthias.kunnen@gmail.com>

Author: MatthiasKunnen

spec/operators/throttleTime-spec.ts

@@ -141,24 +141,34 @@ describe('throttleTime operator', () => {
 });
 
 describe('throttleTime(fn, { leading: true, trailing: true })',()=>{
-asDiagram('throttleTime(fn, { leading: true, trailing: true })')('should immediately emit the first value in each time window',()=>{
+asDiagram('throttleTime(fn, { leading: true, trailing: true })')('should immediately emit the first and last values in each time window',()=>{
 conste1=hot('-a-xy-----b--x--cxxx--|');
 conste1subs='^ !';
-constt=time('----| ');
+constt=time('----| ');
 constexpected='-a---y----b---x-c---x-|';
 
 constresult=e1.pipe(throttleTime(t,rxTestScheduler,{leading: true,trailing: true}));
 
 expectObservable(result).toBe(expected);
 expectSubscriptions(e1.subscriptions).toBe(e1subs);
 });
+
+it('should emit the value if only a single one is given',()=>{
+conste1=hot('-a--------------------|');
+constt=time('----| ');
+constexpected='-a--------------------|';
+
+constresult=e1.pipe(throttleTime(t,rxTestScheduler,{leading: true,trailing: true}));
+
+expectObservable(result).toBe(expected);
+});
 });
 
 describe('throttleTime(fn, { leading: false, trailing: true })',()=>{
-asDiagram('throttleTime(fn, { leading: false, trailing: true })')('should immediately emit the first value in each time window',()=>{
+asDiagram('throttleTime(fn, { leading: false, trailing: true })')('should immediately emit the last value in each time window',()=>{
 conste1=hot('-a-xy-----b--x--cxxx--|');
 conste1subs='^ !';
-constt=time('----| ');
+constt=time('----| ');
 constexpected='-----y--------x-----x-|';
 
 constresult=e1.pipe(throttleTime(t,rxTestScheduler,{leading: false,trailing: true}));
@@ -167,16 +177,26 @@ describe('throttleTime operator', () => {
 expectSubscriptions(e1.subscriptions).toBe(e1subs);
 });
 
-asDiagram('throttleTime(fn, { leading: false, trailing: true })')('should emit the last throttled value when complete',()=>{
+it('should emit the last throttled value when complete',()=>{
 conste1=hot('-a-xy-----b--x--cxx|');
 conste1subs='^ !';
-constt=time('----| ');
+constt=time('----| ');
 constexpected='-----y--------x----(x|)';
 
 constresult=e1.pipe(throttleTime(t,rxTestScheduler,{leading: false,trailing: true}));
 
 expectObservable(result).toBe(expected);
 expectSubscriptions(e1.subscriptions).toBe(e1subs);
 });
+
+it('should emit the value if only a single one is given',()=>{
+conste1=hot('-a--------------------|');
+constt=time('----| ');
+constexpected='-----a----------------|';
+
+constresult=e1.pipe(throttleTime(t,rxTestScheduler,{leading: false,trailing: true}));
+
+expectObservable(result).toBe(expected);
+});
 });
 });

src/internal/operators/throttleTime.ts

@@ -133,6 +133,9 @@ class ThrottleTimeSubscriber<T> extends Subscriber<T> {
 this.add(this.throttled=this.scheduler.schedule<DispatchArg<T>>(dispatchNext,this.duration,{subscriber: this}));
 if(this.leading){
 this.destination.next(value);
+}elseif(this.trailing){
+this._trailingValue=value;
+this._hasTrailingValue=true;
 }
 }
 }