How do I test a Node.JS Script that is not a module and runs as soon as it's called?

Question

I like to use Javascript as a replacement for bash scripts.

Assuming a contrived script called start.js that is run using node start.js:

const shelljs = require("shelljs")

if (!shelljs.which("serve")) {
  shelljs.echo("'serve' is missing, please run 'npm ci'")
  process.exit(1)
}

shelljs.exec("serve -s build -l 3000")

How do I test that:

1. If serve isn't available then serve -s build -l 3000 is never called and program exits with code 1.
2. If serve is available then serve -s build -l 3000 is called.

I don't mind mocking "shelljs", process.exit or anything else.

My main issue is figuring out how to require or import such a functionless and moduleless file in a test suite and get it to run once on each separate test with the mocks present without actually turning this into a CommonJS/ES6 module.

Answer 1

Just mock shelljs module, and spy process.exit function.

describe("start.js", () => {
  let shelljs
  let exitSpy

  beforeEach(() => {
    jest.mock("shelljs", () => {
      return {
        exec: jest.fn(),
        which: jest.fn(),
        echo: jest.fn(),
      }
    })
    shelljs = require("shelljs")
    exitSpy = jest.spyOn(process, "exit").mockImplementation(() => {})
  });

  afterEach(() => {
    jest.resetModules()
    jest.resetAllMocks()
  })

  it("should execute process.exit with code is 1 when 'serve' is not existed", () => {
    shelljs.which.mockReturnValue(false)

    require("./start")

    expect(shelljs.which).toHaveBeenCalledWith("serve");
    expect(shelljs.echo).toHaveBeenCalledWith("'serve' is missing, please run 'npm ci'")
    expect(exitSpy).toHaveBeenCalledWith(1)
    // expect(shelljs.exec).toHaveBeenCalled() // can not check like that, exitSpy will not "break" your code, it will be work well if you use if/else syntax
  });

  it("should execute serve when 'serve' is existed", () => {
    shelljs.which.mockReturnValue(true)

    require("./start")

    expect(shelljs.which).toHaveBeenCalledWith("serve");
    expect(shelljs.echo).not.toHaveBeenCalled()
    expect(exitSpy).not.toHaveBeenCalled()
    expect(shelljs.exec).toHaveBeenCalledWith("serve -s build -l 3000")
  });
})

Another way to make sure the production code with be break when process.exit is called. Mock exit function throw an Error, then expect shelljs.exec will not be called

describe("start.js", () => {
  let shelljs
  let exitSpy

  beforeEach(() => {
    jest.mock("shelljs", () => {
      return {
        exec: jest.fn(),
        which: jest.fn(),
        echo: jest.fn(),
      }
    })
    shelljs = require("shelljs")
    
    exitSpy = jest.spyOn(process, "exit").mockImplementation(() => {
      throw new Error("Mock");
    })
  });

  afterEach(() => {
    jest.resetModules()
    jest.resetAllMocks()
  })

  it("should execute process.exit with code is 1 when 'serve' is not existed", () => {
    shelljs.which.mockReturnValue(false)

    expect.assertions(5)
    try {
      require("./start")
    } catch (error) {
      expect(error.message).toEqual("Mock")
    }

    expect(shelljs.which).toHaveBeenCalledWith("serve");
    expect(shelljs.echo).toHaveBeenCalledWith("'serve' is missing, please run 'npm ci'")
    expect(exitSpy).toHaveBeenCalledWith(1)
    expect(shelljs.exec).not.toHaveBeenCalled()
  });

  it("should execute serve when 'serve' is existed", () => {
    shelljs.which.mockReturnValue(true)

    require("./start")

    expect(shelljs.which).toHaveBeenCalledWith("serve");
    expect(shelljs.echo).not.toHaveBeenCalled()
    expect(exitSpy).not.toHaveBeenCalled()
    expect(shelljs.exec).toHaveBeenCalledWith("serve -s build -l 3000")
  });
})