feat(mapTo): implement mapTo

src/operator/MapToOperator.ts

@@ -0,0 +1,38 @@
+import {InternalListener} from '../InternalListener';
+import {Operator} from '../Operator';
+import {Stream} from '../Stream';
+import {emptyListener} from '../utils/emptyListener';
+
+export class Proxy<T, R> implements InternalListener<T> {
+  constructor(public out: Stream<R>,
+              public op: MapToOperator<T, R>) {
+  }
+
+  _n(t: T) {
+    this.out._n(this.op.projectedValue);
+  }
+
+  _e(err: any) {
+    this.out._e(err);
+  }
+
+  _c() {
+    this.out._c();
+  }
+}
+
+export class MapToOperator<T, R> implements Operator<T, R> {
+  public proxy: InternalListener<T> = emptyListener;
+
+  constructor(public projectedValue: R,
+              public ins: Stream<T>) {
+  }
+
+  _start(out: Stream<R>): void {
+    this.ins._add(this.proxy = new Proxy(out, this));
+  }
+
+  _stop(): void {
+    this.ins._remove(this.proxy);
+  }
+}