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cell0.js

function forward(receiver, metaobject, ...methods) {
  methods.forEach(function(methodName) {
    receiver[methodName] = (...args) => metaobject[methodName](...args);
  });

  return receiver;
}

function delegate(receiver, metaobject, ...methods) {
  methods.forEach(function(methodName) {
    receiver[methodName] = (...args) =>
      metaobject[methodName].apply(receiver, args);
  });

  return receiver;
}

const portfolio = (function() {
  const investments = Symbol();

  return {
    [investments]: [],
    addInvestment(investment) {
      this[investments].push(investment);
    },
    netWorth() {
      return this[investments].reduce(
        function(acc, investment) {
          return acc + investment.value;
        },
        0,
      );
    },
  };
})();

const investor = forward({}, portfolio, 'addInvestment', 'netWorth');
investor.addInvestment({ type: 'art', value: 10 });
investor.addInvestment({ type: 'art', value: 20 });
investor.netWorth();

const banker = forward({}, portfolio, 'addInvestment', 'netWorth');
banker.addInvestment({ type: 'stocks', value: 100 });
banker.addInvestment({ type: 'stocks', value: 200 });
banker.netWorth();