Fix field type detection

Fixes #25

Author: marcusolsson

src/datasource.ts

@@ -151,10 +151,15 @@ export class DataSource extends DataSourceApi<JsonApiQuery, JsonApiDataSourceOpt
  * @returns the detected field type and potentially converted values.
  */
 export const detectFieldType = (values: any[]): [FieldType, any[]] => {
+  if (values.every(_ => _ === null)) {
+    return [FieldType.string, values];
+  }
   // If all values are valid ISO 8601, the assume that it's a time field.
-  const isValidISO = values.filter(value => value).every(value => value.length >= 10 && isValid(parseISO(value)));
+  const isValidISO = values
+    .filter(value => value !== null)
+    .every(value => value.length >= 10 && isValid(parseISO(value)));
   if (isValidISO) {
-    return [FieldType.time, values.map(_ => parseISO(_).valueOf())];
+    return [FieldType.time, values.map(_ => (_ !== null ? parseISO(_).valueOf() : null))];
   }
 
   const isNumber = values.every(value => typeof value === 'number');

src/detectFieldType.test.ts

@@ -23,3 +23,24 @@ test('iso8601 gets parsed as time', () => {
     [1136214373000, 1136214433000],
   ]);
 });
+
+test('nullable iso8601 gets parsed as time', () => {
+  expect(detectFieldType(['2006-01-02T15:06:13Z', null])).toEqual(['time', [1136214373000, null]]);
+});
+
+test('all zeros gets parsed as number', () => {
+  expect(detectFieldType([0, 0, 0])).toEqual(['number', [0, 0, 0]]);
+  expect(detectFieldType([0, 0, 1])).toEqual(['number', [0, 0, 1]]);
+
+  expect(detectFieldType([false, false, false])).toEqual(['boolean', [false, false, false]]);
+  expect(detectFieldType([false, false, true])).toEqual(['boolean', [false, false, true]]);
+});
+
+test('all false gets parsed as boolean', () => {
+  expect(detectFieldType([false, false, false])).toEqual(['boolean', [false, false, false]]);
+  expect(detectFieldType([false, false, true])).toEqual(['boolean', [false, false, true]]);
+});
+
+test('all null gets parsed as string', () => {
+  expect(detectFieldType([null, null])).toEqual(['string', [null, null]]);
+});