SyntaxError: Missing initializer in const declaration

Question

I am writing a simple test using react-native-testing-library (my first steps with that library) in my react native expo app. But I am getting a confused error coming from somewhere inside react-native code base itself. Either there is something wrong with my code or there is a bug with react-native-testing-library npm library.

Here is simple jest test:

describe("AppTitle", () => {
  it("should display applicaton title", () => {
    const { getByText } = render(<AppTitle />);
    expect(getByText('App Name')).toBeTruthy();
  });
});

And here is the simple <AppTitle /> component (just a View and a Text)

export const AppTitle = () => {
  return (
    <View>
      <Text>App Name</Text>
    </View>
  );
};

But I am getting this error when I run the test:

...../Utilities/warnOnce.js:15

const warnedKeys: {[string]: boolean} = {};
      ^^^^^^^^^^

SyntaxError: Missing initializer in const declaration

at ScriptTransformer.transformAndBuildScript (node_modules/@jest/transform/build/ScriptTransformer.js:471:17)
at ScriptTransformer.transform (node_modules/@jest/transform/build/ScriptTransformer.js:513:25)
at Object.<anonymous> (node_modules/react-native/Libraries/react-native/react-native-implementation.js:14:18)
...

This is a simple and straight forward template. Any help from a react-native + react-native-testing-library would be appreciate.

react: 16.8.3
react-native: fork from Expo 33
jest-expo: "^33.0.2"
react-native-testing-library": "1.7.0"

how did you set up TypeScript compiling? seems like your TS type declaration goes untransformed so JS interpreter suppose : to be just a label or something like that — skyboyer
– skyboyer, Commented Jul 23, 2019 at 13:47
@skyboyer I created this project with "expo init app_name". I have not ejected and I have messed up any setting. The new expo cli tool builds the app with babel7 which has built-in support for typescript. Maybe I should some preset in the babel.config.js...I don't know... — TheSoul
– TheSoul, Commented Jul 24, 2019 at 10:05
you need to install/setup ts-jest or babel-jest or check if it's configured if already installed levelup.gitconnected.com/… beware that TS support for Jest and for building are independent so even if one works another still may be misconfigured — skyboyer
– skyboyer, Commented Jul 24, 2019 at 10:11
Thanks for the suggestion. I installed ts-jest and configured jest.config. But now I get this error: SyntaxError: Unexpected token < (on the line for render(<AppTitle />). It looks like the .tsx file has not been transpiled. I continue to investigate — TheSoul
– TheSoul, Commented Jul 24, 2019 at 10:58

Andreea · Accepted Answer · 2019-07-29 13:06:44Z

20

I resolved this added "preset": "react-native" in jest.config.js file

answered Jul 29, 2019 at 13:06

Andreea

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2 Comments

Highriser Over a year ago

Sadly this didn't help me. I am still stuck at the same error as above :(

haleonj Over a year ago

As per the Jest documentation, this is now supposed to be included by default in the package.json file

error-try-again · Accepted Answer · 2025-02-17 19:52:08Z

3

This can sometimes happen if the location of the babel preset library has moved or has been updated in a major update migration e.g. RN 74 -> 77. One way you might fix this is by updating the babel.config.js or equivalant preset line from:

module.exports = {
    presets: ['module:metro-react-native-babel-preset'],
};

to

module.exports = {
    presets: ['module:@react-native/babel-preset']
};

Run react-native start --reset-cache and try again

answered Feb 17 at 19:52

error-try-again

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1 Comment

Omatt Mar 27 at 8:17

This solution worked for me. The issue seems to have been caused by my update on babel-preset during my migration from 0.74 to 0.78

Mahmut Gundogdu · Accepted Answer · 2019-10-15 11:31:10Z

2

I am using expo in my project. I have same issue. i forgot to add "preset": "jest-expo" to package.json. i added then the problem solved.

 "jest": { "preset": "jest-expo" },

edited Oct 15, 2019 at 11:31

answered Oct 15, 2019 at 11:23

Mahmut Gundogdu

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