How to throw an error when an attribute is changed?

Question

I want to make sure an error gets thrown when an attribute of an object gets changed outside the class. Here's how I tried to do it:

class Example {
  constructor(index) {
    this.index = index;

    Object.defineProperty(this, 'index', {
      set() {
        throw new AssertionError("can't set attribute");
      }
    });
  }
}

class AssertionError extends Error {
  constructor(message) {
    super();
    this.name = "AssertionError";
    this.message = message;
  }
}


let example = new Example(5);
console.log(example.index); //prints undefined instead of 5
example.index = 10; // I want to throw an AssertionError here

The error gets thrown just like I wanted, but the index value is undefined. I still want to be able to change the attribute inside of the class, but I want to prevent the attribute from changing outside of the class.

Answer 1

You redefine the property with the call to defineProperty. You should give it a getter:

Object.defineProperty(this, 'index', {
  get() { return index; },
  set() {
    throw new AssertionError("can't set attribute");
  }
});

Any given property name can only be used once; a property has to either be a plain property or a property with getter/setter functions.