I'm trying to mock node-fetch with jest for my azure functions. In the test I have the following:

index.test.ts

jest.mock("node-fetch");
import fetch from "node-fetch";
const {Response} = jest.requireActual("node-fetch");

// Setup code here...

const expectedResult: User = {
        user_id: "1",
        email: "[email protected]",
        name: "testUser",
        nickname: "test",
        picture: "pic",
        app_metadata: {
            email: "[email protected]"
        }
    };
    (fetch as jest.MockedFunction<typeof fetch>).mockReturnValue(new Response(JSON.stringify(expectedResult)));

When I call it I'm doing the following:

index.ts

const options = {
                method: 'PATCH',
                headers: { "Content-Type": 'application/json', authorization: `Bearer ${accessToken}`},
                body: body
            };

const userResponse = await fetch(usersEndpoint, options);
const jsonResult = await userResponse.json();
context.res = {
                body: jsonResult
            };

When it hits the "await userResponse.json()" I get "body used already for" error. I have another test that is set up in a similar manner which works so I'm not sure why it's saying the body is used up from the await fetch call. Any help would be appreciated.

I'm trying to mock node-fetch with jest for my azure functions. In the test I have the following:

index.test.ts

jest.mock("node-fetch");
import fetch from "node-fetch";
const {Response} = jest.requireActual("node-fetch");

// Setup code here...

const expectedResult: User = {
        user_id: "1",
        email: "[email protected]",
        name: "testUser",
        nickname: "test",
        picture: "pic",
        app_metadata: {
            email: "[email protected]"
        }
    };
    (fetch as jest.MockedFunction<typeof fetch>).mockReturnValue(new Response(JSON.stringify(expectedResult)));

When I call it I'm doing the following:

index.ts

const options = {
                method: 'PATCH',
                headers: { "Content-Type": 'application/json', authorization: `Bearer ${accessToken}`},
                body: body
            };

const userResponse = await fetch(usersEndpoint, options);
const jsonResult = await userResponse.json();
context.res = {
                body: jsonResult
            };

When it hits the "await userResponse.json()" I get "body used already for" error. I have another test that is set up in a similar manner which works so I'm not sure why it's saying the body is used up from the await fetch call. Any help would be appreciated.

• javascript
• node.js
• typescript
• jestjs
• node-fetch

2 Answers 2

Response object is supposed to be used once per request while mocked fetch returns the same object for multiple requests. Also, it should return a promise of a response, not a response itself.

A correct way to mock it is:

fetch.mockImplementation(() => Promise.resolve(
  new Response(JSON.stringify(expectedResult))
));

It's unnecessary to use Response and follow the restrictions it imposes, especially since there's no native Response in Node.

It can be:

fetch.mockResolvedValue({
  json: jest.fn(() => expectedResult)
});

My problem was I had a call to another function that used fetch which was resolving my mock implementation. Once I mocked that return value:

 (fetch as jest.MockedFunction<typeof fetch>).mockReturnValue(new Response(JSON.stringify(expectedResult)));

ended up working.

@Estus Flask's answer also ended up working.