Test Nextjs <Link /> with Jest + Testing-Library

Question

When I'm testing the <Link /> behavior, expecting it to redirect to a certain route, there's a TypeError (Cannot read property 'push' of null).

This is the component I'm currently testing:

import React from "react"
import Link from "next/link"

const Sandbox = () => {
  return (
    <div>
      <Link href="/about">
        <a data-testid="mytest">Click Me</a>
      </Link>
    </div>
  )
}

export default Sandbox

And this is the test I'm running:

import React from "react"
import { render, fireEvent } from "@testing-library/react"
import { useRouter } from "next/router"
import Sandbox from ".."

jest.mock("next/router", () => ({
  useRouter: jest.fn(),
}))

describe("Sandbox", () => {
  it.only("should navigate accordingly", () => {
    const push = jest.fn()
    useRouter.mockImplementationOnce(() => ({
      asPath: "/",
      push,
    }))

    const { getByTestId } = render(<Sandbox />)

    const mytest = getByTestId("mytest")
    fireEvent.click(mytest)
    expect(push).toHaveBeenCalledWith("/about")
  })
})

I believe that I've mocked everything I need, so I don't really understand why the router can't actually "push". What am I missing here?

Answer 1

Turns out the module that has to be mocked is slightly different (https://github.com/vercel/next.js/issues/7479#issuecomment-797811147)

Had this problem myself so, based on one of my tests, this should run:

import { render, fireEvent, screen } from "@testing-library/react"
import { useRouter } from "next/router"
import Sandbox from ".."
  
jest.mock("next/dist/client/router", () => ({
   useRouter: jest.fn(),
}))
 
describe("Sandbox", () => {
  const mockPush = jest.fn(() => Promise.resolve(true));
   
  beforeAll(() => {
    useRouter.mockReturnValue({
      asPath: "/",
      query: {},
      push: mockPush,
      prefetch: () => Promise.resolve(true)
    })
  })
    
  test("should navigate accordingly", () => {
    
    render(<Sandbox />)
   
    const mytest = screen.getByTestId("mytest")
    fireEvent.click(mytest)
   
    expect(mockPush).toHaveBeenCalledWith("/about", expect.anything(), expect.anything())
  })
})

I've added expect.anything(), because the push function could be called with its other parameters (https://nextjs.org/docs/api-reference/next/router#routerpush)