module-mutator.js

import * as fs from "fs";

let exports;

// reflective API takes environment descriptors and uses revealing constructor pattern
// to expose mutators to the creator of the module instance object
var mod = new Reflect.Module({
  // initialized mutable (includes "uninitialized" var)
  x: { value: undefined },
  y: { value: 42 },

  // uninitialized mutable (let, class)
  z: { },

  // initialized immutable (const)
  PI: { value: Math.PI, const: true },

  // uninitialized immutable (const)
  TAU: { const: true },

  // re-export
  readFile: { module: fs, import: "readFile" }
}, function(mutator) { exports = mutator; });

// mutator object uses setters to write through to the module's encapsulated environment
console.log(mod.y); // 42
exports.y = 43;
console.log(mod.y); // 43