Mock a const defined in a function using Jest

Question

My project is a React application using Jest.

I have to test a function in which I am defining a const. The const is set using an imported service.

I want to mock the const in order to test the if-else statement

Here is the file I want to test:

import { authenticationService } from "../_services/authentication.service";

export const handleHeaders = (isFile = false) => {
    const currentUser = authenticationService.currentUserValue;
    if (currentUser && currentUser.access_token) {
        return isFile
            ? {
                Authorization: `Bearer ${currentUser.access_token}`,
                "Content-Disposition": "multipart/form-data"
            }
            : {
                Authorization: `Bearer ${currentUser.access_token}`,
                "Content-Type": "application/json"
            };
    } else {
        return { "Content-Type": "application/json" };
    }
};

I tried to mock auth authentication service like below

import { handleHeaders } from "../handle-headers";

jest.mock("../../_services/authentication.service", () => ({
  currentUserValue: { access_token: "some token" }
}));

describe("handleHeaders", () => {
  it("should return headers with Authorization if 
currentUser.access_token is defined", () => {
    expect(handleHeaders()).toStrictEqual({
      Authorization: "Bearer some token",
      "Content-Type": "application/json"
    });
  });
});

But I get TypeError: Cannot read property 'currentUserValue' of undefined When the function tries to define const currentUser.

Thanks for your help

May you show your effort to mock it? Have you looked at the jest docs? — evolutionxbox

psilocybin psilocybin · Accepted Answer · 2020-01-07T10:49:59

You can use ES6 mocks: https://jestjs.io/docs/en/es6-class-mocks

Syntax:

jest.mock('../_services/authentication.service', () => ({
    __esModule: true,
    authenticationService: {
        currentUserValue: { access_token: 'some token' }
    }
}))

However, remember to call authenticationService.mockClear() in your beforeEach block to reset this between tests.

Edit: Because authenticationService isn't the default export, we need to mock it this way.

Mock a const defined in a function using Jest

1 Answers