example .

.eslintrc.json

{
    /** 
    * ESLint: http://eslint.org/docs/user-guide/configuring
    */

    "globals": {
        "$": true
    },

    // "env:" supplies predefined global variables
    "env": {
        "browser": true,
        "es6": true,
        "node": true,
        "mocha": true,
        "mongo": true
    },
    // our configuration extends the recommended base configuration
    "extends": ["eslint:recommended", "react-app"]

    // define the type of file `script` or `module` for ES6 Modules
    "parserOptions": {
        "sourceType": "script",
        "ecmaFeatures": {
            "jsx": true
        }
    },
    //ESLint rules: Severity Levels: off = 0 | warn = 1 | error = 2
    "rules": {
        "strict": ["error", "safe"],   //prefer `'use-strict';` pragma
        "eqeqeq":"error",              //prefer strict equality `===`
        "no-console": "warn",          //allows but warn about console like `console.log()`
        "no-unused-vars": "warn",      //warns about unused variables
        "no-eval": "error",            //disallows `eval()` usage
        "indent": ["error", 2],        //enforce 2 space indents (not tabs)        
        "quotes": ["error", "single"], //prefer single quotes over double quotes
        "semi": ["error", "always"],    //enforce semi-colon usage
        "react/jsx-uses-react": "error",
        "react/jsx-uses-vars": "error"
    }
}