How to use class variables inside constructor in JavaScript?

Question

Why am I not able to access variables inside the constructor?

class Dir {
  refs = {};

  constructor(dir, label, reverse, clockwise) {
    this.dir = dir;

    // error: refs is not defined
    Dir.prototype.refs[label] = { ref: this, reverse, clockwise };
    // this wont work either
    Dir.refs[label] = { ref: this, reverse, clockwise };
  }
}

// test
const dirU = new Dir([0, -1], 'U', 'D', 'R');

refs is accessible outside the constuctor:

class Dir {
  refs = {};
  constructor(dir, label, reverse, clockwise) {
    this.dir = dir;
  }
}
const dirU = new Dir([0, -1], 'U', 'D', 'R');
console.log(dirU.refs);
// will print '{}'

This will solve it, but it is much less convient:

class Dir {
  constructor(dir, label, reverse, clockwise) {
    this.dir = dir;
    if (!(label in Dir.prototype)) Dir.prototype.refs = {};
    Dir.prototype.refs[label] = { ref: this, reverse, clockwise };
  }
}

How can I access class variable inside the constructor? Is this even possible in javascript?

clarification:i want refs to remember all previous instances with a dictionary of labels. I want the Dir class to hold a dictionary of all of his instances. in python this is how you would write it, but in javascript it doesn't work.

Dir.prototype.refs[label] -> this.refs[label]. If you want to interact with the instance, it's just this. You're not interacting with the prototype or statically with the class. — VLAZ
– VLAZ, Commented May 18, 2021 at 10:31
no, i want refs to remember all previous instances with a dictionary of labels — Eliav Louski
– Eliav Louski, Commented May 18, 2021 at 10:33

Siddharth · Accepted Answer · 2021-05-18 10:40:54Z

3

Make refs static. Then use Dir.refs. That is:

class Dir {
  static refs = {};

  constructor(dir, label, reverse, clockwise) {
    this.dir = dir;
    Dir.refs[label] = { ref: this, reverse, clockwise };
  }
}

answered May 18, 2021 at 10:40

Siddharth

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3 Comments

Eliav Louski Over a year ago

great! thanks! will accept it in 3 minutes.

Peter Seliger Over a year ago

@EliavLouski ... how about the concern you were raising in your Q. of not exposing the references into public? ... "... refs is accessible outside the constructor ...". This above provided approach does not prevent it. The only approach which does fully cover this requirement was to use additional encapsulation.

Eliav Louski Over a year ago

@PeterSeliger actually, I want it to be accessible publicly

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