text: // Example for CommonJS Module Format for export (used in Node)
  
  // Implementation Detail
  const _amount = new WeakMap();
  
  // public interface
  class Invoice {
      constructor(amount) {
          _amount.set(this, amount);
      }
  
      generate() {
          console.log('Generating Invoice for amount' + _amount.get(this));
      }
  }
  
  // commonjs format of export (used in Node)
  module.exports = Invoice;