jest.spyOn(module, 'method', 'get') failed as method not declared configurable

This seems to be an edge case around mocking or spyOn a module built with Babel, while it may be related to babel module export settings, but would like to get some insights here first to see if there’s anything I’ve missed or anything we can do on Jest side.

deps versions:

    "babel-cli": "^6.26.0",
    "babel-core": "^6.26.3",
    "babel-plugin-transform-object-rest-spread": "^6.26.0",
    "babel-preset-env": "^1.7.0",
    "jest": "^23.5.0",

Had a depedency dep-package built with babel with the following .babelrc:

{
  "presets": [
    [
      "env", { "targets": { "node": "6.1.0" } }
    ]
  ],
  "plugins": [
    "transform-object-rest-spread"
  ]
}

in test.js

import * as depPackage from 'dep-package';

jest.mock('dep-package');

module mocks would work

import * as depPackage from 'dep-package';

depPackage.compose = jest.fn();

import * as depPackage from 'dep-package';

jest.spyOn(depPackage, 'compose');

this would throw error TypeError: Cannot set property compose of [object Object] which has only a getter

import * as depPackage from 'dep-package';

jest.spyOn(depPackage, 'compose', 'get');

this would throw error compose is not declared configurable

Not sure what is causing the difference between module mock with jest.mock() and manual mock =jest.fn() here?

[AnilGayakwad]

@zhenyulin give a try with this,

jest.mock('dep-package', () => ({ ...jest.requireActual('dep-package'), })); const cwpDetectionAgency = require('dep-package');

instead of doing import * as depPackage from 'dep-package';

This has helped me, any thing which is out side of the current code base we have to do this.

[faiwer]

Finally I got this. It’s an unrobust, but working solution, that monkey-patches Object.defineProperty in the setup file.

In setupTests.js:

const { defineProperty } = Object;
Object.defineProperty = function(object, name, meta) {
  if (meta.get && !meta.configurable) {
    // it might be an ES6 exports object
    return defineProperty(object, name, {
      ...meta,
      configurable: true, // prevent freezing
    });
  }

  return defineProperty(object, name, meta);
};

In tests helpers:

export const unfreezeImport = <T,>(module: T, key: keyof T): void => {
  const meta = orDie(Object.getOwnPropertyDescriptor(module, key));
  const getter = orDie(meta.get);

  const originalValue = getter() as T[typeof key];
  let currentValue = originalValue;
  let isMocked = false;

  Object.defineProperty(module, key, {
    ...meta,
    get: () => (isMocked ? currentValue : getter()),
    set(newValue: T[typeof key]) {
      isMocked = newValue !== originalValue;
      currentValue = newValue;
    },
  });
};

in tests:

import * as someModule from 'someModule';

unfreezeImport(someModule, 'someProperty');

it('someTest', () => {
  jest.spyOn(someModule, 'someProperty').mockImplementation(...);
  // test code
});

May be it will help someone. Also this unfreezeImport method can be united with the patched defineProperty in setupTests.js file.