In this program:

class Example {

  #privateMember = 123;

  // these are fine  
  addNumber (n) { return this.#privateMember   n; }
  doAddNumber (n) { return this.addNumber(n); }

  // "cannot read private member #privateMember from an 
  // object whose class did not declare it"
  #operations = { add: this.addNumber };
  operate (n) { return this.#operations.add(n); }

}

const ex = new Example();
console.log(ex.addNumber(77));
console.log(ex.doAddNumber(77));
console.log(ex.operate(77));

Calling addNumber works fine, so does doAddNumber, but calling operate yields the error:

TypeError: Cannot read private member #privateMember from an object whose class did not declare it
    at Object.addNumber [as add] (<anonymous>:11:17)
    at Example.operate (<anonymous>:20:29)
    at <anonymous>:27:16
    at dn (<anonymous>:16:5449)

It's runnable at https://playcode.io/1033251.

I can't make any sense of this error because:

1. addNumber works fine, so its not a syntax error or a typo at least.
2. doAddNumber works fine, so its not a problem calling functions from other functions.
3. operate just calls addNumber which, from (1), works fine.
4. this is an object whose class declares #privateMember... I mean, this is an Example and I can see that it's declared in Example. It's right there... I typed it myself...

I found TypeError: Cannot read private member from an object whose class did not declare it but I can't understand how it applies, if it applies.

I can't figure out what's going on here. Why doesn't operate work even though addNumber and doAddNumber do?

In my real code (this is just a minimal example), I am trying to use a dictionary like #operations to hold implementations of a number of various algorithms for performing a task, indexed by a string ID, where the string algorithm ID is specified to the constructor. This is also convenient because I can get the keys from this dictionary to provide a list of valid algorithm IDs without having to duplicate that list anywhere. Now, I can just switch it to an if statement and make sure I keep the queryable list up to date as well, but I can't understand why this doesn't work.

CodePudding user response：

It's occuring because private fields can only be accessed from within the class that declares them, and object literals are not considered part of the class, so, basically if you create a constructor method in your class may solve your problem.

constructor () {
   this.#operations = { add: this.addNumber };
}

I just summarize the answer but try to read to understand with more details why this occur: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields

CodePudding user response：

When you set #operations.addNumber and then call it, this === #operations, not ex, which fails because ex.#operarions.#privateMember does not exist.

If I change your Example class with

#operations = { add: this.addNumber.bind(this)};

then ex.#operations.add runs with this === ex and your code returns

> 200
> 200
> 200

CodePudding user response：

Here's a working example of what I believe you're after...

class Example {

  #privateMember = 123;

  // these are fine  
  addNumber (n) { return this.#privateMember   n; }
  doAddNumber (n) { return this.addNumber(n); }

  // "cannot read private member #privateMember from an 
  // object whose class did not declare it"
  #operations = { add: this.addNumber };
  operate (n) { return this.#operations.add.call(this, n); }

}

const ex = new Example();
console.log(ex.addNumber(77));
console.log(ex.doAddNumber(77));
console.log(ex.operate(77));

In short, in your code the #operations method is simply creating an object with a reference to the Example class method of addNumber, with no association to any object. So, when attempting to make use of this reference, you need to pass an object...

Ie, you're essentially defining...

#operations = { add: Example.prototype.addNumber }

In the proposed solution, the operate method invokes the reference to the addNumber method in the #operations object, but of course has to pass an object to addNumber, and does so by performing a call( this, ...).