javascript - How to mock a library in node_modules? - Stack Overflow

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I'm trying to write a test for code that uses node-forge. For some reason, the test hangs when I call forge.md.sha256.create();:

  import forge from "node-forge";

  const privateKey = "foo";
  const storagePin = "bar";

  const md = forge.md.sha256.create();
  md.update(privateKey + storagePin);

  const metadataKey = md.digest().toHex();

As a workaround, I am trying to mock the implementation of that method so that it just returns a hardcoded string:

import forge from "node-forge";
jest.mock("node-forge");

forge.mockImplementation(() => {
  return {
    md: {
      sha256: {
        create: () => {
          return {
            update: () => {},
            digest: () => {
              toHex: () => "foobar";
            }
          };
        }
      }
    }
  };
});


// tests

However, my tests keep failing:

TypeError: _nodeForge2.default.mockImplementation is not a function

  at Object.<anonymous> (src/redux/epics/authentication-epic.test.js:20:27)
      at new Promise (<anonymous>)
  at Promise.resolve.then.el (node_modules/p-map/index.js:46:16)
  at processTicksAndRejections (internal/process/next_tick.js:81:5)

The weird thing is, this strategy works perfectly fine when I try to mock my own files.

What is the proper way of mocking a 3rd party library?

I'm trying to write a test for code that uses node-forge. For some reason, the test hangs when I call forge.md.sha256.create();:

  import forge from "node-forge";

  const privateKey = "foo";
  const storagePin = "bar";

  const md = forge.md.sha256.create();
  md.update(privateKey + storagePin);

  const metadataKey = md.digest().toHex();

As a workaround, I am trying to mock the implementation of that method so that it just returns a hardcoded string:

import forge from "node-forge";
jest.mock("node-forge");

forge.mockImplementation(() => {
  return {
    md: {
      sha256: {
        create: () => {
          return {
            update: () => {},
            digest: () => {
              toHex: () => "foobar";
            }
          };
        }
      }
    }
  };
});


// tests

However, my tests keep failing:

TypeError: _nodeForge2.default.mockImplementation is not a function

  at Object.<anonymous> (src/redux/epics/authentication-epic.test.js:20:27)
      at new Promise (<anonymous>)
  at Promise.resolve.then.el (node_modules/p-map/index.js:46:16)
  at processTicksAndRejections (internal/process/next_tick.js:81:5)

The weird thing is, this strategy works perfectly fine when I try to mock my own files.

What is the proper way of mocking a 3rd party library?

• javascript
• reactjs
• jestjs

Share Improve this question Follow edited May 15, 2023 at 18:35 isherwood 61.2k1616 gold badges121121 silver badges170170 bronze badges asked Apr 4, 2019 at 20:38 bigpotatobigpotato 27.6k6262 gold badges194194 silver badges360360 bronze badges 1

• Try mocking first, then importing the module. jest.mock("node-forge") followed by import forge from "node-forge" – Ricardo Leon Commented Apr 4, 2019 at 22:05

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2 Answers 2

Sorted by: Reset to default 6

did you try like this? More about this here.

jest.mock('node-forge', () => ({
  md: {
    sha256: {
      create: () => ({
        update: () => {},
        digest: () => ({
          toHex: () => 'foobar'
        }),
      }),
    },
  },
}));

The default export isn't a function so Jest auto-mocking doesn't replace the default export with a mock function...

...but the default export is an object.

From Exploring ES6:

...while you can’t change the values of imports, you can change the objects that they are referring to.

So you can just set the md property on the object to your mock:

import forge from 'node-forge';
jest.mock('node-forge');

const toHex = jest.fn(() => 'foobar');
const digest = jest.fn(() => ({ toHex }));
const update = jest.fn();

forge.md = {  // <= set the md property to your mock
  sha256: {
    create: jest.fn(() => ({
      update,
      digest
    }))
  }
};

test('code uses the mock', () => {
  require('./path to your code');  // <= the mock will be used in the required code
  expect(forge.md.sha256.create).toHaveBeenCalled();  // Success!
  expect(update).toHaveBeenCalledWith('foobar');  // Success!
  expect(digest).toHaveBeenCalled();  // Success!
  expect(toHex).toHaveBeenCalled();  // Success
});

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