example1.js

// node-handler.js
const fs = require('fs');

export const handle = () => { 
  fs.readFileSync(...);
  ...
};

// browser-handler.js
export const handle = () => { ... };

// index.js
const nodeHandler = require('./node-handler').handle;
const browserHandle = require('./browser-handler').handle;
const isNode = require('./config').isNode;

const handle = isNode ? nodeHandler : browserHandle;