Mock Contract.allowance from ethers.js using Jest

I am working on writing tests for a project, and somewhere in the project there is a function containing the following code:

const erc20 = new Contract(
  tokenAddress,
  ERC20_ABI,
  provider,
);
const approved = await erc20.allowance(walletAddress, ChainData.address);

I want to mock the erc20.allowance function call so that the unittest I am writing for that function does not depend on this call, but I don't know how to do this.

I tried writing the following code:

jest.spyOn(Contract.prototype, 'allowance').mockImplementation(() => {
  console.log('mocking success!');
  return BigNumber.from('100000');
});

but I get an error when running it:

Property `allowance` does not exist in the provided object

I also tried using jest.mock to mock the whole Contract class but it does not work:

describe('getAllowance', () => {
  jest.mock('ethers', () => {
    return jest.fn().mockImplementation(() => {
      class MockContract {
        async allowance() {
          console.log('mocking success!');
          return BigNumber.from('100000');
        }
      }

      return {
        ...originalEthers,
        Contract: MockContract,
      };
    });
  });

  it('should return the allowance if everything is alright', async () => {
    expect.assertions(1);

    const response = await approveService.getAllowance(
      1116,
      tokenAddress,
      'some wallet address',
    );

    expect(response.allowance).toBe('100000');
  });
});

Solution

I was finally able to mock another module using jest. The problem was that I needed to put jest.mock in the global scope of the test file for it to work. This led to another problem of course: the mock would not be test case specific and would be there for all of test cases in a file. It is a much easier problem though and can be fixed by a couple of ways, like for example moving test cases to different files or writing more complex mock implementations that would cover the other test cases as well.