TypeError: Cannot set property logger of #<Object> which has only a getter

[jmikulka]

v7 Regression

First check out: https://new.babeljs.io/docs/en/next/v7-migration.html
Also a partial upgrade tool: https://github.com/babel/babel-upgrade

Potential Commit/PR that introduced the regression
N/A

Describe the regression
Running jest tests with the babel-preset-env@6 pass:

yarn run v1.7.0
warning package.json: No license field
$ jest
 PASS  ./deps.test.js
  Test
    ✓ logger has been called (5ms)

  console.log deps.test.js:4
    { logger: [Function] }

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        1.277s
Ran all test suites.
✨  Done in 2.33s.

After upgrade to @babel/preset-env@7, the tests fail:

yarn run v1.7.0
warning package.json: No license field
$ jest
 FAIL  ./deps.test.js
  Test
    ✕ logger has been called (10ms)

  ● Test › logger has been called

    TypeError: Cannot set property logger of #<Object> which has only a getter

       6 |   const mockLogger = jest.fn();
       7 |   beforeEach(() => {
    >  8 |     jest.spyOn(deps, 'logger').mockImplementation(mockLogger);
         |          ^
       9 |   });
      10 |
      11 |   it('logger has been called', () => {

      at ModuleMockerClass.spyOn (node_modules/jest-mock/build/index.js:706:26)
      at Object.<anonymous> (deps.test.js:8:10)

  ● Test › logger has been called

    expect(jest.fn()).toHaveBeenCalledWith(expected)

    Expected mock function to have been called with:
      ["test"]
    But it was not called.

      11 |   it('logger has been called', () => {
      12 |     deps.logger('test');
    > 13 |     expect(mockLogger).toHaveBeenCalledWith('test');
         |                        ^
      14 |   });
      15 | });
      16 |

      at Object.<anonymous> (deps.test.js:13:24)

  console.log deps.test.js:4
    { logger: [Getter] }

  console.log logger.js:1
    LOGGER [ 'test' ]

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        1.119s
Ran all test suites.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Input Code
Clone https://github.com/jmikulka/babel_preset_env_regression and run:

BABEL_ENV=old yarn test # test passes, uses babel-preset-env

BABEL_ENV=new yarn test # test fails, uses @babel/preset-env

Babel Configuration (.babelrc, package.json, cli command)

// .babelrc.js
module.exports = {
  presets: [process.env.BABEL_ENV === 'old' ? 'env' : '@babel/preset-env'],
};

Expected behavior/code
The tests with the new preset should not fail.

Environment

• Babel version(s): [v7.0.0-beta.54]
• Node/npm version: [Node 8.11.3/npm 5.6.0]
• OS: [OSX 10.13.5]
• Monorepo [no]
• How you are using Babel: [babel-jest]

[babel-bot]

Hey @jmikulka! We really appreciate you taking the time to report an issue. The collaborators
on this project attempt to help as many people as possible, but we're a limited number of volunteers,
so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack
community that typically always has someone willing to help. You can sign-up here
for an invite.

[jmikulka]

Based on a slack discussion https://babeljs.slack.com/archives/C0DFJT81H/p1533733732000482 I've found a solution.

In deps.test.js replace

import * as deps from './deps';
const mockLogger = jest.fn();
jest.spyOn(deps, 'logger').mockImplementation(mockLogger);

with

const mockLogger = jest.fn();
jest.mock('./deps', () => ({ logger: mockLogger }));
const deps = require('./deps');

However, by mocking ./deps this way, we lose any other named/default exports.

[echjordan]

I'm getting the same error after upgrading to babel 7. In addition to this getter error, I'm getting an error that seems to be common when minified files are not properly separated by semicolons, (intermediate value).concat is not a function and wanted to mention here in case they're related.
I used babel-upgrade, have followed the documentation for upgrading.
Using:

    "@babel/core": "^7.1.2",
    "@babel/helper-plugin-utils": "^7.0.0",
    "@babel/plugin-proposal-class-properties": "^7.0.0",
    "@babel/plugin-proposal-decorators": "^7.1.2",
    "@babel/plugin-proposal-optional-chaining": "^7.0.0",
    "@babel/polyfill": "^7.0.0",
    "@babel/preset-env": "^7.1.0",
    "@babel/register": "^7.0.0"```

[echjordan]

Any update on this issue?

[beshanoe]

Encountered the same issue in my tests. I think, mocking the dependency is the only right approach, cause patching exported object is pretty unsafe and can lead to unexpected behaviour. So from that point of view it's good that babel 7 started enforcing this.
@jmikulka in order to not to lose other named exports you can do the following:

jest.mock("./deps", () => ({
  ...jest.requireActual("./deps"),
  logger: jest.fn()
}));

test("test something", () => {
  const deps = require("./deps");

  // do something

  expect(deps.logger).toHaveBeenCalled();
});

That's how I solved it in my tests :)

[nicolo-ribaudo]

Yah that is a good solution. I'm closing this issue because it's not a bug; the specification mandates that esmodules may not be mutated from the outside.