adds type guards for time unit ids

src/components/date_picker/super_date_picker/relative_options.ts

@@ -1,4 +1,4 @@
-import { TimeUnitId, RelativeOption } from '../types';
+import { RelativeOption, TimeUnitId, TimeUnitFromNowId } from '../types';
 
 export const relativeOptions: RelativeOption[] = [
   { text: 'Seconds ago', value: 's' },
@@ -18,12 +18,20 @@ export const relativeOptions: RelativeOption[] = [
   { text: 'Years from now', value: 'y+' },
 ];
 
-export const relativeUnitsFromLargestToSmallest = relativeOptions
-  .map(({ value }) => {
-    return value;
-  })
-  .filter(value => {
-    // NOTE_TO_SELF(dimitri): a TypeScript assertion couldn't hurt here, instead of a cast
-    return !value.includes('+');
-  })
-  .reverse() as TimeUnitId[];
+const timeUnitIds = relativeOptions
+  .map(({ value }) => value)
+  .filter(value => value.includes('+')) as TimeUnitId[];
+
+export const isTimeUnitId = (value: string): value is TimeUnitId =>
+  timeUnitIds.includes(value as TimeUnitId);
+
+const timeUnitFromNowIds = relativeOptions
+  .map(({ value }) => value)
+  .filter(value => !value.includes('+')) as TimeUnitFromNowId[];
+
+export const isTimeUnitFromNowId = (
+  value: string
+): value is TimeUnitFromNowId =>
+  timeUnitFromNowIds.includes(value as TimeUnitFromNowId);
+
+export const relativeUnitsFromLargestToSmallest = timeUnitIds.reverse();