feat(debounce): emit pending value on completion

Emit any pending value, then complete, if a source stream completes
before a debounce interval closes. Clear pending value upon
completion. Support falsy emission values such as `false` and 0.

Resolve #257.

src/extra/debounce.ts

@@ -1,9 +1,10 @@
-import {Operator, Stream} from '../index';
+import {Operator, Stream, NO} from '../index';
 
 class DebounceOperator<T> implements Operator<T, T> {
   public type = 'debounce';
   public out: Stream<T> = null as any;
   private id: any = null;
+  private t: any = NO;
 
   constructor(public dt: number,
               public ins: Stream<T>) {
@@ -32,9 +33,11 @@ class DebounceOperator<T> implements Operator<T, T> {
     const u = this.out;
     if (!u) return;
     this.clearInterval();
+    this.t = t;
     this.id = setInterval(() => {
       this.clearInterval();
       u._n(t);
+      this.t = NO;
     }, this.dt);
   }
 
@@ -49,6 +52,8 @@ class DebounceOperator<T> implements Operator<T, T> {
     const u = this.out;
     if (!u) return;
     this.clearInterval();
+    if (this.t != NO) u._n(this.t);
+    this.t = NO;
     u._c();
   }
 }

tests/extra/debounce.ts

@@ -2,6 +2,7 @@
 /// <reference types="node" />
 import xs, {Listener, Producer} from '../../src/index';
 import debounce from '../../src/extra/debounce';
+import fromDiagram from '../../src/extra/fromDiagram';
 import * as assert from 'assert';
 
 describe('debounce (extra)', () => {
@@ -29,4 +30,34 @@ describe('debounce (extra)', () => {
     };
     stream.addListener(listener);
   });
+
+  it('should emit any pending value upon completion', (done: any) => {
+    const stream = fromDiagram('-1----2-|').compose(debounce(50));
+    const expected = [1, 2];
+    stream.addListener({
+      next: (x: number) => {
+        assert.equal(x, expected.shift());
+      },
+      error: (err: Error) => done(err),
+      complete: () => {
+        assert.strictEqual(expected.length, 0);
+        done();
+      },
+    });
+  });
+
+  it('should not emit value upon completion if no pending value', (done: any) => {
+    const stream = fromDiagram('-1----2-------|').compose(debounce(50));
+    const expected = [1, 2];
+    stream.addListener({
+      next: (x: number) => {
+        assert.equal(x, expected.shift());
+      },
+      error: (err: Error) => done(err),
+      complete: () => {
+        assert.strictEqual(expected.length, 0);
+        done();
+      },
+    });
+  });
 });