开玩笑：react-native-device-info：NativeModule.RNDeviceInfo 是 null

Question

I am doing my very first jest test for a React Native 0.68, jest/29.0 and nodes 18. The test case is a unit test on a function:

import helper from "./helper";

it ('describe isHex ', () => {
    const num = "13ad";
    expect(helper.isHex(num)).toBe(true);
    const num1 = "12z";
    expect(helper.isHex(num1)).toBe(false);
})

Here is the function isHex in helper.js:

import DeviceInfo from 'react-native-device-info';
...
isHex : (num) => {
    return Boolean(num.match(/^0x[0-9a-f]+$/i))
  },

The yarn jest throws error about DeviceInfo (not being used in function being tested) below:

● Test suite failed to run

    react-native-device-info: NativeModule.RNDeviceInfo is null. To fix this issue try these steps:

The app runs fine in emulator without error. The problem seems with jest only.

Here is part of package.json:

"devDependencies": {
    "@babel/core": "^7.18.9",
    "@babel/plugin-proposal-private-methods": "^7.18.6",
    "@babel/plugin-transform-flow-strip-types": "^7.18.9",
    "@babel/runtime": "^7.18.9",
    "@react-native-community/eslint-config": "^3.0.3",
    "babel-jest": "^28.1.3",
    "eslint": "^8.20.0",
    "jest": "^29.0.0",
    "metro-react-native-babel-preset": "^0.71.3",
    "react-test-renderer": "17.0.2"
  },
  "jest": {
    "preset": "react-native",
    "cacheDirectory": "./cache",
    "setupFiles": [
      "<rootDir>/jest.setup.js"
    ],
    "moduleFileExtensions": [
      "ts",
      "tsx",
      "js",
      "jsx",
      "json",
      "node"
    ],
    "transform": {
      "^.+\\.(js|jsx)$": "babel-jest"
    },
    "transformIgnorePatterns": []
  }

Answer 1

this is where you need to mock the package, after imports in your test file, do this

 jest.useFakeTimers();

    // this should work
    jest.mock('react-native-device-info', () => () => jest.fn());

    // or you can try this
     jest.mock('react-native-device-info', () => ({
  
  default: jest.fn(),

}));

hope it helps. feel free for doubts

Answer 2

Do make sure that you close the Previous Bundle of App.