CODEWITHSUNDEEP
javascriptes6-classes6-proxy
Uday Hiwarale
Uday Hiwarale

Reputation: 4157

How can I modify a constructor so that it return a proxy that prevents adding new properties to the object?

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?

Upvotes: 29

Views: 20838

Answers (2)

Vahid Hallaji
Vahid Hallaji

Reputation: 7447

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.

Upvotes: 43

Joshua Skrzypek
Joshua Skrzypek

Reputation: 458

You can do this without using Proxies at all.

In your class constructor you can define the password property like this:

constructor(options, schema) {
    this.name = options.name;
    this.email = options.email;
    Object.defineProperty(this, 'password', {
        configurable: false, // no re-configuring this.password
        enumerable: true, // this.password should show up in Object.keys(this)
        value: options.password, // set the value to options.password
        writable: false // no changing the value with this.password = ...
    });
    // whatever else you want to do with the Schema
}

You can find more information about how to use this on the MDN's Object.defineProperty() page.

Upvotes: 19

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