// webpack.config.js
const webpack = require('webpack');
const ModuleMappingPlugin = require('module-mapping-webpack-plugin');

module.exports = {
  // ...
  plugins: [
    new ModuleMappingPlugin({
      './foo.js': './foo-spartez.js',
      // ...
    })
  ]
};

// foo.js
export default () => console.log('Hello World!');

// foo-spartez.js
export default () => console.log('Hello Spartez!');

// index.js
import foo from './foo';
foo(); // → 'Hello Spartez!';