Implement startWith() operator

spec/operators/startWith-spec.js

@@ -0,0 +1,20 @@
+/* globals describe, it, expect */
+var Rx = require('../../dist/cjs/Rx');
+var Observable = Rx.Observable;
+
+describe('Observable.prototype.startWith()', function () {
+  it('should start an obsevable with given value', function (done) {
+    var source = 'source'
+    var init = 'init';
+    var expected = [init, source];
+
+    var i = 0;
+    Observable.just(source)
+    .startWith(init)
+    .subscribe(function (x) {
+      expect(x).toBe(expected[i++]);
+    }, null, function () {
+      done();
+    });
+  });
+});

src/Observable.ts

@@ -122,6 +122,7 @@ export default class Observable<T> {
   toArray: () => Observable<T[]>;
   scan: <R>(project: (acc: R, x: T) => R, acc?: R) => Observable<R>;
   reduce: <R>(project: (acc: R, x: T) => R, acc?: R) => Observable<R>;
+  startWith: <T>(x: T) => Observable<T>;
 
   filter: (predicate: (x: T) => boolean, ix?: number, thisArg?: any) => Observable<T>;
   skip: (count: number) => Observable<T>;
@@ -134,6 +135,6 @@ export default class Observable<T> {
 
   publish: () => ConnectableObservable<T>;
   multicast: (subjectFactory: () => Subject<T>) => ConnectableObservable<T>;
-  
+
   catch: (selector: (err: any, source: Observable<T>, caught: Observable<any>) => Observable<any>) => Observable<T>;
 }

src/Rx.ts

@@ -76,12 +76,14 @@ import mapTo from './operators/mapTo';
 import toArray from './operators/toArray';
 import scan from './operators/scan';
 import reduce from './operators/reduce';
+import startWith from './operators/startWith';
 
 observableProto.map = map;
 observableProto.mapTo = mapTo;
 observableProto.toArray = toArray;
 observableProto.scan = scan;
 observableProto.reduce = reduce;
+observableProto.startWith = startWith;
 
 import filter from './operators/filter';
 import take from './operators/take';

src/operators/startWith.ts

@@ -0,0 +1,8 @@
+import Observable from '../Observable';
+import ScalarObservable from '../observables/ScalarObservable';
+import concat from './concat';
+
+export default function startWith<T>(x: T): Observable<T> {
+  const init = new ScalarObservable(x);
+  return concat.call(init, null, <Observable<T>>this);
+}