docs(operators): add documentation for defaultIfEmpty

src/operator-docs/conditional/defaultIfEmpty.ts

@@ -1,6 +1,67 @@
 import { OperatorDoc } from '../operator.model';
 
 export const defaultIfEmpty: OperatorDoc = {
-  'name': 'defaultIfEmpty',
-  'operatorType': 'conditional'
+  name: 'defaultIfEmpty',
+  operatorType: 'conditional',
+  signature: 'public defaultIfEmpty(defaultValue: any): Observable',
+  marbleUrl: 'http://reactivex.io/rxjs/img/defaultIfEmpty.png',
+  parameters: [
+    {
+      name: 'defaultValue',
+      type: 'any',
+      attribute: 'optional default: null',
+      description: 'The default value used if the source Observable is empty.'
+    }
+  ],
+  shortDescription: {
+    description: `Emits a given value if the source Observable completes without emitting any
+        <span class="markdown-code">next</span> value, otherwise mirrors the source Observable.
+    `,
+    extras: [
+      {
+        type: 'Tip',
+        text: `
+            If the source Observable turns out to be empty, then this operator will emit a default value.
+        `
+      }
+    ]
+  },
+  walkthrough: {
+    description: `
+      <p>
+        <span class="markdown-code">defaultIfEmpty</span> emits the values emitted by the source
+        Observable or a specified default value if the source Observable is empty (completes
+        without having emitted any <span class="markdown-code">next</span> value).
+      </p>
+    `
+  },
+  examples: [
+    {
+      name: `If no clicks happen in 3 seconds, then emit 'no clicks'`,
+      code: `
+        import { fromEvent } from 'rxjs/observable/fromEvent';
+        import { interval } from 'rxjs/observable/interval';
+        import { defaultIfEmpty, takeUntil } from 'rxjs/operators';
+
+        const clicks = fromEvent(document, 'click');
+        const result = clicks.pipe(
+          takeUntil(interval(3000)),
+          defaultIfEmpty('no clicks')
+        );
+
+        result.subscribe(x => console.log(x));
+
+        /*
+        Example console output
+        no clicks
+        */
+      `,
+      externalLink: {
+        platform: 'JSBin',
+        url: 'http://jsbin.com/verovam/embed?js,console,output'
+      }
+    }
+  ],
+  relatedOperators: ['empty', 'last'],
+  additionalResources: []
 };