Instruction on "Flow" installation

Copy files from previous step

cp -r ../009-rest-client/calc/ . && \
cd calc && \
yarn

Add Flow preset for babel

yarn add --dev @babel/preset-flow

Add preset to config/config.js

extraBabelPresets: [
    "@babel/flow",
  ],

Install flow-bin

yarn add --dev flow-bin

Init flow

flow init

Add ignoring .*/node_modules/.* in .flowconfig

Check flow status

flow

Add // @flow to first lines of src/client/calculator.js, src/client/index.js and src/client/random.js

Check flow status again. There should be errors. Fix them.

flow

Check ui still works

yarn start

Add checking before commit in package.json

"husky": {
    "hooks": {
      "pre-commit": "npm run prettier && npm run lint-staged && flow"
    }
  }