Primitive closure implementation in JS to illustrate that closure is "just" an object

closure.js

class RootScope {
  constructor() {
    this.map = new Map();
  }

  add(name, value) {
    return this.map.set(name, value);
  }

  set(name, value) {
    return this.map.set(name, value);
  }

  get(name) {
    if (this.map.has(name)) {
      return this.map.get(name);
    }

    throw new Error(`ReferenceError: ${name} is not defined`);
  }
}

class Scope {
  constructor(parentScope) {
    this.parent = parentScope;
    this.map = new Map();
  }

  add(name, value) {
    this.map.set(name, value);
  }

  set(name, value) {
    if (this.map.has(name)) {
      return this.map.set(name, value);
    }

    return this.parent.set(name, value);
  }

  get(name) {
    if (this.map.has(name)) {
      return this.map.get(name);
    }

    return this.parent.get(name);
  }
}

class Closure {
  constructor(parentScope, fn) {
    this.scope = new Scope(parentScope);
    this.fn = fn;
  }

  call(...args) {
    return this.fn.call(null, this.scope, ...args);
  }
}

// - - - - - - - - - - - - - - - - - - - - - - - - - - - -

function _inner(scope) {
  return scope.get("x") + scope.get("y");
}

function _main(scope) {
  scope.add("y", 5);

  const inner = new Closure(scope, _inner);

  return inner;
}

const globalScope = new RootScope();
globalScope.set("x", 7);

const outer = new Closure(globalScope, _main);

const closure = outer.call();
console.log(closure.call());
globalScope.set("x", 10);
console.log(closure.call());

console.log("done");