Installing Flow depends on whether you are working with Babel (as would be the case for client-side browser code) or not (as you would do for server-side code). We will see how to deal with Node starting in Chapter 3, Developing with Node; here, we'll just consider Babel. 

To start, execute the following command to get the needed Flow packages, including the Babel and ESLint ones:

npm install flow-bin babel-preset-flow eslint-plugin-flowtype --save-dev

Then, add the "flow" preset for Babel in package.json: 

"babel": {
    "presets": ["env", "flow"] 
},

Add some lines to the ESLint configuration, also in package.json:

"eslintConfig": {
    "parserOptions": {
        "ecmaVersion": 2017,
        "sourceType": "module"
    },
    "env": {
        "browser": true,
        "node": true
    },
    "parser": "babel-eslint",
    "extends": ["eslint:recommended", "plugin:flowtype/recommended"],
    "plugins": ["flowtype"],
    "rules": {
        .
        .
        .
    }
},

Add a "flow" script in package.json:

"scripts": {
    "build": "babel src -d out",
    "flow": "flow",
    .
    .
    .
},

Finally, perform npm run flow init to initialize Flow, only once, to create a .flowconfig file with information that will be used by the Flow process. (See https://flow.org/en/docs/config/ for more information on this file.)