Create React App 2.0

config-overrides.js for CRA 1.0

const path = require('path');

module.exports = function override(config, env) {
    config.resolve.alias = Object.assign({}, config.resolve.alias, {
        environment: path.join(__dirname, 'src', 'environments', process.env.NODE_ENV)
    });

    return config;
};

config-overrides.js for CRA 2.0

const path = require('path');
const {override, addWebpackAlias} = require('customize-cra');

module.exports = override(
    addWebpackAlias({
        environment: path.join(__dirname, 'src', 'environments', process.env.NODE_ENV)
    })
);