Jest/React/Mobx: TypeError mobxReact.observer is not a function

Question

I'm trying to run a simple Jest test against React/Mobx code that just render a route and checks if it was rendered. I don't use classes and Mobx decorators. I receive an error on every place where I wrap my component with Mobx observer function:

const MyComp: React.FC = observer(() => {...})

My Jest config:

module.exports = {
    "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(ts|tsx|js|jsx)$",
    "moduleFileExtensions": [
        "ts",
        "tsx",
        "js"
    ],
    "roots": ['<rootDir>'],
    "modulePaths": ['<rootDir>'],
    "moduleDirectories": [
        ".",
        "src",
        "node_modules"
    ],
    "setupFiles": [
        "raf/polyfill",
        "<rootDir>/jest.setup.js"
    ],
    "snapshotSerializers": [
        "enzyme-to-json/serializer"
    ],
    "testPathIgnorePatterns": [
        "<rootDir>/node_modules/"
    ],
    "transform": {
        "\\.(ts|tsx)$": "ts-jest",
        "\\.js$": "babel-jest",
        '^.+.(css|styl|less|sass|scss|png|jpg|ttf|woff|woff2|mp4)$': 'jest-transform-stub',
        '^.+\\svg': 'jest-svg-transformer',
        '^.+\\inline.svg': 'jest-svg-transformer'
    },

    "moduleNameMapper": {
        "mobx": "<rootDir>/node_modules/mobx",
        '^.+.(css|styl|less|sass|scss|png|jpg|ttf|woff|mp4|woff2)$': 'jest-transform-stub',
        '^.+\\svg': 'jest-svg-transformer',
        '^.+\\inline.svg': 'jest-svg-transformer'
    },

    "coverageReporters": [
        "html", "text"
    ]
}

My test case:

describe('General:', () => {
    test('landing page renders ok', async () => {
        render((
            <ThemeProvider theme={ theme }>
                { renderRoutes(routes) }
            </ThemeProvider>
        ), { wrapper: BrowserRouter });
        await screen.findByText('Correct text here')
    })
});

I tried keep everything the same and just removed mobx observer wrapper from a component and all works fine with absolutely the same setup.

• mobx: 6.4.0
• mobx-react: 7.5.0
• jest: 26.6.3

Answer 1

The Jest documentation about moduleNameMapper states that:

Note: If you provide module name without boundaries ^$ it may cause hard to spot errors. E.g. relay will replace all modules which contain relay as a substring in its name: relay, react-relay and graphql-relay will all be pointed to your stub.

You have mobx in your moduleNameMapper which will stub out both mobx and mobx-react, which is not what you want. You should be able to get rid of the error by removing that.