How can I debug private class fields in Node.js with VS Code?

Question

How do I check the contents of the private fields of a class instance during runtime, debugging a Node app in VS Code?

It should be something basic but somehow seems not to be possible...

• VS Code's Javascript Debugger doesn't displays the private fields of objects
• I tried util.inspect, but Node folks decided to not make it work.

If not possible, which are the workarounds?

I'm using:

• Node 15.3.0
• VS Code 1.51.1

Answer 1

Update:

VS Code's built-in javascript debugger did not support debugging of private class fields at the time of this question. It does now, since version 1.56.0 (April 2021).

---

Old answer:

VS Code's built-in javascript debugger (ms-vscode.js-debug) does not support private class fields yet. (there's a feature request for it on github)

However, v8 does have a (currently in experimental state) method for reading private class fields. I've made a proof of concept project (leonardoraele/private-field-inspect) that uses Node's inspector API to programatically open a debug session at runtime to read the contents of the variable. (this does not interrupt the runtime execution)

It works, but with a few caveats.

Usage:

import inspect from '../path/to/private-field-inspect';

class Subject
{
    publicValue = 'not so secret';
    #secretValue = 'my secret';
}

const subject = new Subject();

inspect(subject)
    .then(console.debug);

Output:

{ publicValue: 'not so secret', '#secretValue': 'my secret' }

---

The workaround above is not satisfactory for me, so I think it's better to avoid using private class fields if possible, until they are properly supported by debuggers. Instead, use Symbols to hide private variables.

Example:

// my-class.js (or .msj)
const PRIVATE = {
    SECRET_VAL = Symbol('#secretVal');
};

export default class Subject {
    // ms-vscode.js-debug can read it, but other modules can't
    [PRIVATE.SECRET_VAL]: 'my secret';

    doSomething()
    {
        const secret = this[PRIVATE.SECRET_VAL];
        // ...
    }
}