recordもどき

record.js

function toRecord(origin) {

  function check(lock, prop) {
    if (!lock) {
      throw new Error('Changes outside $mutate');
    } else if (prop === '$mutate') {
      throw new Error('$mutate can never be changed');
    }
  }

  function convert(src) {
    let lock = false;
    let copy;

    src.$mutate = fn => {
      copy = Object.assign({},  src);
      lock = true;
      fn();
      lock = false;
      return convert(copy);
    };

    return new Proxy(src, {
      set(_, prop, value) {
        check(lock, prop);
        copy[prop] = value;
        return true;
      },
      deleteProperty(_, prop) {
        check(lock, prop);
        delete copy[prop];
        return true;
      }
    });
  }

  return convert(Object.assign({},  origin));
}

const origin = {
  count: 0,
  name: 'test'
};

console.log('origin to record1');
const record1 = toRecord(origin);
console.log('origin', JSON.stringify(origin));
console.log('record1', JSON.stringify(record1));

try {
  // direct
  record1.count = 1;
} catch (e) {
  console.log(e);
}

console.log('record1 to record2 ( record1 | count = 1 )');
const record2 = record1.$mutate(() => {
  record1.count = 1;
});
console.log('origin', JSON.stringify(origin));
console.log('record1', JSON.stringify(record1));
console.log('record2', JSON.stringify(record2));

console.log('record2 to record3 ( record2 | count = 2, name = test2 )');
const record3 = record2.$mutate(() => {
  record2.count = 2;
  record2.name = 'test2';
});
console.log('origin', JSON.stringify(origin));
console.log('record1', JSON.stringify(record1));
console.log('record2', JSON.stringify(record2));
console.log('record3', JSON.stringify(record3));

console.log('record1 to record4 ( delete name property )');
const record4 = record1.$mutate(() => {
  delete record1.name;
});
console.log('origin', JSON.stringify(origin));
console.log('record1', JSON.stringify(record1));
console.log('record2', JSON.stringify(record2));
console.log('record3', JSON.stringify(record3));
console.log('record4', JSON.stringify(record4));