First

src/first.js

@@ -0,0 +1,18 @@
+/**
+ * Returns the first element to be returned by an Observable
+ *
+ * @param {Observable} input Input Observable
+ * @return {Observable} New Observable
+ */
+
+const first = input => {
+  let out;
+  input.subscribe(x => {
+    if (!out) out = x;
+  });
+  return out;
+};
+
+first._name = 'first';
+
+export default first;

src/index.js

@@ -8,6 +8,7 @@ import defer from './defer';
 import delay from './delay';
 import error from './error';
 import filter from './filter';
+import first from './first';
 import flatMap from './flatMap';
 import forEach from './forEach';
 import fromEvent from './fromEvent';
@@ -37,6 +38,7 @@ const addAll = (target = Observable) => {
     debounce,
     delay,
     filter,
+    first,
     flatMap,
     forEach,
     map,
@@ -67,6 +69,7 @@ export {
   delay,
   error,
   filter,
+  first,
   flatMap,
   forEach,
   fromEvent,

test/first.js

@@ -0,0 +1,11 @@
+import { expect } from 'chai';
+import first from '../src/first';
+import apiCheck from './common/apiCheck';
+
+describe('(First) defer', () => {
+  apiCheck(first);
+
+  it('emits the first element of an Observable', () => {
+    expect(first(Observable.of(1, 2, 3))).to.eql(1);
+  });
+});