In TypeScript field initializer override properties set by super constructor

[tbuschto]

Problem description

class Foo extends Composite {
  private _bar = 1;
  constructor(properties) {
    super(properties);
  }
  set bar(value) {
    this._bar = value;
  }
  get bar() { return this._bar; }
}

console.log(new Foo({bar: 2}).bar);

This prints "1"

Expected behavior

Should print "2"

Environment

Any tabris version
any TypeScript version
any Platform

[tbuschto]

Seems unfixable. I was hoping the new public fields proposal (https://github.com/tc39/proposal-class-fields) would fix this as soon as it is adopted by TypeScript, but this does not seem to be the case, the fields are still initialized after the super call, overriding anything set there.

The workaround is to use set(properties), but this can not be enforced. An attempt was made to disallow setting properties via super() via runtime checks, but this turned out to be a technical nightmare.

Another "solution" would be to change the declaration of the constructor parameter so it never allows unknown properties (known as excess property check in TypeScript), making super(properties) an error if 'properties' has has entries not known to the super type. However, this also seems impossible at this time, though there are plans to support "exact" types: microsoft/TypeScript#28464

At this point I'm not sure that there is anything we could do except document the issue.