Jest

foo.spec.ts

const fooFn = jest.fn().mockImplementation(() => Promise.resolve('bar'));
const barFn = jest.fn();
jest.mock('./foo', () => ({ foo: fooFn, bar: barFn });
import Foo from './foo';

describe('foo module tests', () => {
  it('runs foo', () => {
    const foo = Foo();
    expect(fooFn).toHaveBeenCalled();
    expect(barFn).toHaveBeenCalledTimes(0);
  });
});

foo.ts

class Foo {
  constructor() {
    console.log('new Foo created');
    this.foo();
  }

  foo() {
    console.log('Foo's foo method called');
  }

  bar() {
    console.log('Foo's bar method called');
  }
}

export default Foo;