Is there any way to mock private functions with Jest?

If you want to mock a private function, try to use the prototype.

For example, you need to mock privateFunction of the following class:

export class Module {
    public publicFunction() {
        // do something
        this.privateFunction();
        // do something
    }

    private privateFunction() {
        // do something
    }
}  

So you should use Module.prototype in the jest.spyOn function.

import { Module } from './my-module';

describe('MyModule', () => {
  it('tests public function', () => {
    // Arrange
    const module = new Module()
    const myPrivateFunc = jest.spyOn(Module.prototype as any, 'privateFunction');
    myPrivateFunc.mockImplementation(() => {});

    // Act
    module.publicFunction();

    // Assert
    expect(myPrivateFunc).toHaveBeenCalled();
  });
});