getting-started.md

Getting started with Jest

https://jestjs.io/

Install Jest

npm install --save-dev jest
npm i -g jest

configuration

# jest.config.js

module.exports = {
  verbose: true,
  preset: "react-native",
  moduleFileExtensions: [
    "ts",
    "tsx",
    "js",
    "jsx",
    "json",
    "node"
  ],
  transformIgnorePatterns: [
    "node_modules/(?!(@react-native|react-native" 
    + "|react-native-vector-icons|react-native-linear-gradient)/)"
  ]
}

transformIgnorePatterns

Some react-native libraries ship uncompiled ES6 code. It needs to be compiled before it can be run by Jest.

You can tell Jest to compile those packages by whitelisting them in the transformIgnorePatterns option in the jest config.

The transformIgnorePatterns option (which is an array of Regular-Expressions) is originally meant to exclude files from being compiled, but using (?!(some-dir-name|another-name)) pattern (the (?!...), negative look-ahead), we do tell Jest to exclude anything in node_modules directory, except the names that we did specify.

fixing "cannot find name 'jest'" lint error in typescript

• install @types/jest
• import '@types/jest' in the test file
• add types to `tsconfig.json'

{
  "compilerOptions": {
      "types": ["node", "jest"]
  },
}