Returning ES6 Proxy from the ES6 class constructor

Question

I want user to only set specific properties to an object but as the same time that object should be constructed from custom class.

For example

var row = new Row({
  name : 'John Doe',
  email : '[email protected]'
}, Schema);

row can have methods. But when user is trying to set row.password, they are not allowed.

One way to do it is using new Proxy instead of new Row but then we will loose all cool things we are doing inside Row class. I want new Row to return a proxy object with this reference as a target of proxy.

Anybody have any ideas on this? If you know mongoose, how mongoose is doing it?

Answer 1

If the proxy is certain to happen for you, one possible solution to limit the set functionality is returning an ES6 Proxy instance.

By default, the constructor in javascript returns this object automatically but you could define and return a custom behavior by instantiating a proxy on this as a target. Keep in mind that the set method in proxy should return a boolean value.

MDN: The set method should return a boolean value. Return true to indicate that assignment succeeded. If the set method returns false, and the assignment happened in strict-mode code, a TypeError will be thrown.

class Row {
  constructor(entry) {
    // some stuff

    return new Proxy(this, {
      set(target, name, value) {
        let setables = ['name', 'email'];
        if (!setables.includes(name)) {
          throw new Error(`Cannot set the ${name} property`);
        } else {
          target[name] = value;
          return true;
        }
      }
    });
  }

  get name() {
    return this._name;
  }
  set name(name) {
    this._name = name.trim();
  }
  get email() {
    return this._email;
  }
  set email(email) {
    this._email = email.trim();
  }
}

So, now you are not allowed to set the non-setable properties according to the proxy.

let row = new Row({
  name : 'John Doe',
  email : '[email protected]'
});

row.password = 'blahblahblah'; // Error: Cannot set the password property

It's also possible to have s custom behavior on get method too.

However, beware and take care of overriding the reference that is returned to the calling context.

Note: The sample code has been tested on Node v8.1.3 and modern browsers.