Unable to retrieve private field using a public getter method through a proxy

Question

Unable to retrieve private field using a public getter method through a proxy

When retrieving data from a VueX Module using a Getter, the Object is enclosed within a Proxy that triggers the following error:

TypeError: attempted to get private field on non-instance

when attempting to access a private property with a public getter.

class Example {
  #privateField;

  get privateField() {
    return this.#privateField;
  }
}

computed(){
  getInfoForSpecificItem() {
    const arrayOfInstantiatedExamples = this.$store.getters['getArrayOfInstantiatedExamples'];
    for (let i = 0; i < arrayOfInstantiatedExamples.length; i += 1) {
      // The error occurs when trying to access it in this condition as the Object
      // has been enclosed within a Proxy originating from Vue (as assumed)
      if (arrayOfInstantiatedExamples[i].privateField === 'something') {
        // carries out an action
      }
    }
  }
}

Why am I unable to retrieve the property and how can I extract an instance from the proxy target? Upon logging the Proxy Object, it is evident that the target contains all the information, yet accessing it via the getter doesn't function.

Are there any alternatives besides making the fields public?

javascript vue.js vuex

Answer 1

Answer №1

The answer provided earlier is inaccurate; I cannot confirm its accuracy at the time of writing, but even then, it seems to be incorrect.

#fieldName is a valid syntax in JavaScript classes, indicating that a field is private.

However, a limitation here is that private fields must be declared within the class initializer, as demonstrated. It's not possible to redefine a field with the same name as a getter function. In your code, this issue doesn't seem to arise (as the getter function would need to be get #fieldName()).

Your class should function properly without being wrapped in a proxy, as you have observed.

When wrapped in a Proxy, the object becomes a 'Proxy object' encapsulating your 'myObject.' This means that the object referenced does not actually contain the #myPrivate field (unless the built-in Proxy class includes such a field in its initializer or if you extend the internal Proxy class and define a #myPrivate field there - even then, it would be a distinct #myPrivate variable from the one defined in the class being proxied).

This behavior may be confusing compared to how public fields work on objects. When an object is proxied (or when a class is extended and a new object is instantiated), it belongs to the class (either Proxy or the extended class) and only private #myPrivate fields can be accessed within that class or instance of the class where they are defined. Public fields/properties do not exhibit this hidden behavior and can be accessed easily even when a class is extended using super.myMethod() for overwritten methods.

The existence of #privateFields serves the purpose of information hiding and implementation clarity. allowing developers to clearly distinguish between accessible and internal properties/values. However, it's worth noting that JavaScript being an interpreted language means that interested parties could potentially figure out and replicate access to private values despite efforts to hide them (e.g., through obfuscation or compilation).

It appears that Proxy traps also don't track accesses to private values.

An alternative approach could involve assigning the proxy directly to the class itself and incorporating both trap and standard getter/setter mechanisms for handling private values:

// Implementation example...

By following this method, private fields within the same class can be successfully trapped, although access from different classes or instances remains restricted.

A different technique involves directly specifying property names in the getter/setter functions instead of attempting to universally access private values based on a specific format.

In conclusion, accessing private fields in JavaScript involves deliberate design choices aimed at maintaining data privacy and preventing unintended access. While certain workarounds exist, the core principle of information hiding remains crucial in software development practices.

Answer 2