I have a function as such:

<span style="color:#323232;">export type SendMessageParams = {
</span><span style="color:#323232;">  chatSession?: ChatSession,
</span><span style="color:#323232;">  // ... other params ...
</span><span style="color:#323232;">};
</span><span style="color:#323232;">
</span><span style="color:#323232;">const sendMessage = async ({
</span><span style="color:#323232;">  chatSession,
</span><span style="color:#323232;">  // ... other params ...
</span><span style="color:#323232;">}: SendMessageParams): Promise<void> => {
</span><span style="color:#323232;">  // await chatSession?.sendMessage()
</span><span style="color:#323232;">  // somewhere in implementation
</span><span style="color:#323232;">};
</span><span style="color:#323232;">
</span><span style="color:#323232;">export default sendMessage;
</span>

ChatSession is from @google/generative-ai.

I’d like to mock it in my test file as such:

<span style="color:#323232;">let defaultParams: SendMessageParams;
</span><span style="color:#323232;">
</span><span style="color:#323232;">beforeEach(() => {
</span><span style="color:#323232;">  jest.mock('@google/generative-ai', () => ({
</span><span style="color:#323232;">    ChatSession: {
</span><span style="color:#323232;">      sendMessage: async (content: string) => content,
</span><span style="color:#323232;">    },
</span><span style="color:#323232;">  }));
</span><span style="color:#323232;">  defaultParams = {
</span><span style="color:#323232;">    chatSession: new ChatSession('', ''),
</span><span style="color:#323232;">    // ... other params ...
</span><span style="color:#323232;">  };
</span><span style="color:#323232;">});
</span><span style="color:#323232;">
</span><span style="color:#323232;">afterEach(() => {
</span><span style="color:#323232;">  jest.clearAllMocks();
</span><span style="color:#323232;">});
</span><span style="color:#323232;">
</span><span style="color:#323232;">it('should send message', async () => {
</span><span style="color:#323232;">  // await sendMessage();
</span><span style="color:#323232;">});
</span>

When I run npm run test, I get the error saying:

<span style="color:#323232;"> FAIL  tests/logic/actions/sendMessage.test.ts
</span><span style="color:#323232;">  ● should send message
</span><span style="color:#323232;">
</span><span style="color:#323232;">    ReferenceError: fetch is not defined
</span><span style="color:#323232;">
</span><span style="color:#323232;">      43 |   const sendMessageInner = async (messages: Message[]) => {
</span><span style="color:#323232;">      44 |     setMessageListState(messages);
</span><span style="color:#323232;">    > 45 |     const result = await chatSession?.sendMessage(content);
</span><span style="color:#323232;">         |                    ^
</span><span style="color:#323232;">      46 |     const responseText = result?.response.text();
</span><span style="color:#323232;">      47 |     if (responseText) {
</span><span style="color:#323232;">      48 |       const responseMessage: Message = {
</span><span style="color:#323232;">
</span><span style="color:#323232;">      at makeRequest (node_modules/@google/generative-ai/dist/index.js:246:9)
</span><span style="color:#323232;">      at generateContent (node_modules/@google/generative-ai/dist/index.js:655:28)
</span><span style="color:#323232;">      at node_modules/@google/generative-ai/dist/index.js:890:25
</span><span style="color:#323232;">      at ChatSession.sendMessage (node_modules/@google/generative-ai/dist/index.js:909:9)
</span><span style="color:#323232;">      at sendMessageInner (src/logic/actions/sendMessage.ts:45:20)
</span><span style="color:#323232;">      at src/logic/actions/sendMessage.ts:72:7
</span><span style="color:#323232;">      at sendMessage (src/logic/actions/sendMessage.ts:59:3)
</span><span style="color:#323232;">      at Object.<anonymous> (tests/logic/actions/sendMessage.test.ts:44:3)
</span><span style="color:#323232;">
</span>

…which hints that chatSession.sendMessage method still uses the real implementation instead of mock.

I’d like to know why this happens and what the solution would be.

Thanks in advance.

Environment

• Node 20.11.0 (lts/iron)
• Jest 29.7.0
• @google/generative-ai 0.5.0 (if relevant)