update interval module, add tests

src/helpers/index_timestamp.js

@@ -0,0 +1,38 @@
+import moment from 'moment';
+
+export const intervals = [
+  'year',
+  'month',
+  'week',
+  'day',
+  'hour',
+  'minute'
+];
+
+export default function indexTimestamp(intervalStr) {
+  const index = intervals.indexOf(intervalStr);
+  if (index === -1) throw new Error('Invalid index interval: ', intervalStr);
+
+  const m = moment();
+  m.startOf(intervalStr);
+
+  let dateString;
+  switch (intervalStr) {
+    case 'year':
+      dateString = 'YYYY';
+      break;
+    case 'month':
+      dateString = 'YYYY-MM';
+      break;
+    case 'hour':
+      dateString = 'YYYY-MM-DD-HH';
+      break;
+    case 'minute':
+      dateString = 'YYYY-MM-DD-HH-mm';
+      break;
+    default:
+      dateString = 'YYYY-MM-DD';
+  }
+
+  return m.format(dateString);
+}