refactor(map): throw error when not passed a function

map will now throw an error for users that mistakenly try to use it as mapTo

related: #994

spec/operators/map-spec.js

@@ -19,6 +19,12 @@ describe('Observable.prototype.map()', function () {
     expectSubscriptions(a.subscriptions).toBe(asubs);
   });
 
+  it('should throw an error if not passed a function', function () {
+    expect(function () {
+      Observable.of(1, 2, 3).map('potato');
+    }).toThrow(new TypeError('argument is not a function. Are you looking for `mapTo()`?'));
+  });
+
   it('should map multiple values', function () {
     var a =   cold('--1--2--3--|');
     var asubs =    '^          !';

src/operator/map.ts

@@ -13,6 +13,9 @@ import {errorObject} from '../util/errorObject';
  * @returns {Observable} a observable of projected values
  */
 export function map<T, R>(project: (x: T, ix?: number) => R, thisArg?: any): Observable<R> {
+  if (typeof project !== 'function') {
+    throw new TypeError('argument is not a function. Are you looking for `mapTo()`?');
+  }
   return this.lift(new MapOperator(project, thisArg));
 }