diff --git a/packages/data-sources/file-jsonl/test/index.test.ts b/packages/data-sources/file-jsonl/test/index.test.ts
index eaff939..185c9f0 100644
--- a/packages/data-sources/file-jsonl/test/index.test.ts
+++ b/packages/data-sources/file-jsonl/test/index.test.ts
@@ -20,6 +20,8 @@ describe('prepareResource', () => {
   });
 });
 
+const toJsonl = (dummyItems: unknown[]) => dummyItems.map((i) => JSON.stringify(i)).join('\n')
+
 describe('methods', () => {
   const dummyItems = [
     {
@@ -39,7 +41,7 @@ describe('methods', () => {
     name: v.string(),
   });
   let ds: DataSource<v.Output<typeof schema>>;
-  let mockGenerationStrategy;
+  let mockGenerationStrategy: Mock;
   beforeEach(() => {
     mockGenerationStrategy = vi.fn();
     const r = resource(schema)
@@ -55,9 +57,13 @@ describe('methods', () => {
 
   beforeEach(() => {
     const mockReadFile = readFile as Mock;
-    mockReadFile.mockReturnValueOnce(
-      dummyItems.map((i) => JSON.stringify(i)).join('\n')
-    );
+    mockReadFile.mockReturnValueOnce(toJsonl(dummyItems));
+  });
+
+  let mockWriteFile: Mock;
+  beforeEach(() => {
+    mockWriteFile = writeFile as Mock;
+    mockWriteFile.mockImplementationOnce(() => { /* noop */ });
   });
 
   describe('initialize', () => {
@@ -94,7 +100,7 @@ describe('methods', () => {
 
     describe('getById', () => {
       it('works', async () => {
-        const item = await ds.getById('2');
+        const item = await ds.getById('2'); // ID is always a string because it originates from URLs
         const expected = dummyItems.find((i) => i.id === 2);
         expect(item).toEqual(expected);
       });
@@ -103,6 +109,7 @@ describe('methods', () => {
     describe('getSingle', () => {
       it('works', async () => {
         if (typeof ds.getSingle !== 'function') {
+          // skip if data source doesn't offer this
           return;
         }
         const item = await ds.getSingle();
@@ -111,10 +118,83 @@ describe('methods', () => {
       });
     });
 
-    describe.todo('create');
-    describe.todo('delete');
-    describe.todo('emplace');
-    describe.todo('patch');
+    describe('create', () => {
+      it('works', async () => {
+        const data = {
+          // notice we don't have IDs here, as it is expected to be generated by newId()
+          name: 'foo'
+        };
+        const newItem = await ds.create(data);
+
+        expect(mockWriteFile).toBeCalledWith(
+          expect.any(String),
+          toJsonl([
+            ...dummyItems,
+            data
+          ])
+        );
+        expect(newItem).toEqual(data);
+      });
+    });
+
+    describe('delete', () => {
+      it('works', async () => {
+        await ds.delete('1');
+        expect(mockWriteFile).toBeCalledWith(
+          expect.any(String),
+          toJsonl(dummyItems.filter((d) => d.id !== 1)),
+        );
+      });
+    });
+
+    describe('emplace', () => {
+      it('works', async () => {
+        const data = {
+          id: 2,
+          name: 'foo',
+        };
+        const { id, ...etcData } = data;
+        const newItem = await ds.emplace(
+          id.toString(),
+          etcData,
+        );
+
+        expect(mockWriteFile).toBeCalledWith(
+          expect.any(String),
+          toJsonl(dummyItems.map((d) =>
+            d.id === 2
+              // ID will be defined last, since we are just writing to file, we need strict ordering
+              ? { ...etcData, id }
+              : d
+          )),
+        );
+        expect(newItem).toEqual([data, false]);
+      });
+    });
+
+    describe('patch', () => {
+      it('works', async () => {
+        const data = {
+          id: 2,
+          name: 'foo',
+        };
+        const { id, ...etcData } = data;
+        const newItem = await ds.emplace(
+          id.toString(),
+          etcData,
+        );
+
+        expect(mockWriteFile).toBeCalledWith(
+          expect.any(String),
+          toJsonl(dummyItems.map((d) =>
+            d.id === 2
+              ? { ...etcData, id }
+              : d
+          )),
+        );
+        expect(newItem).toBeDefined();
+      });
+    });
 
     describe('newId', () => {
       it('works', async () => {