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>

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', () =>  {
       const e1 =   hot('-a-xy-----b--x--cxxx--|');
       const e1subs =   '^                     !';
-      const t =  time( '----|                 ');
+      const t =  time( '----|                  ');
       const expected = '-a---y----b---x-c---x-|';
 
       const result = 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', () => {
+      const e1 =   hot('-a--------------------|');
+      const t =   time('----|                  ');
+      const expected = '-a--------------------|';
+
+      const result = 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', () =>  {
       const e1 =   hot('-a-xy-----b--x--cxxx--|');
       const e1subs =   '^                     !';
-      const t =  time( '----|                 ');
+      const t =  time( '----|                  ');
       const expected = '-----y--------x-----x-|';
 
       const result = 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', () => {
       const e1 =   hot('-a-xy-----b--x--cxx|');
       const e1subs =   '^                  !';
-      const t =   time('----|              ');
+      const t =   time('----|               ');
       const expected = '-----y--------x----(x|)';
 
       const result = 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', () => {
+      const e1 =   hot('-a--------------------|');
+      const t =   time('----|                  ');
+      const expected = '-----a----------------|';
+
+      const result = 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);
+      } else if (this.trailing) {
+        this._trailingValue = value;
+        this._hasTrailingValue = true;
       }
     }
   }