wooq lets you mock your JS/TS code for tests.

TODO(me): publish on npm

It requires you to modify existing files by adding annotations allowing code to be mocked. Some people don't like this, but you can set skip outside tests to make the code basically a no-op.

Usage:

const mockable = buildMockable({ skip: !process.env.TEST });

// code.ts
@mockable.class
export class ComplexClass {
  foo() {
    return 100;
  }
}

// test.ts
test('mock class', () => {
  mockable.replaceForTest(ComplexClass, {
    foo() {
      return -999;
    },
  };

  const x = new UsesComplexClassInternally();
  assert.equals(x.whatever(), -999);
});

wooq also has a configurable storage layer (for "what is currently replacing a real class"). This might be useful for 'thread-local' storage in concurrent tests.

wooq supports mocking classes and functions, but TS only has annotation support for classes (something something hoisting rules). So, this works, but might be awkward:

const mockableFunctionThatDefaultReturns123 = mockable.function(() => {
  return 123;
});

wooq is boom but upside-down.