Generated constructor uses this before calling super()

[bpierre]

In certain cases, esbuild can generate a constructor that will use this before super() gets called.

You can reproduce it with this TS code and --target=es2019:

export class A {}

export class B extends A {
  #e: string
  constructor(c: { d: any }) {
    super()
    this.#e = c.d ?? 'test'
  }
  f() {
    return this.#e
  }
}

Which transforms into:

var __accessCheck = (obj, member, msg) => {
  if (!member.has(obj))
    throw TypeError("Cannot " + msg);
};
var __privateGet = (obj, member, getter) => {
  __accessCheck(obj, member, "read from private field");
  return getter ? getter.call(obj) : member.get(obj);
};
var __privateSet = (obj, member, value, setter) => {
  __accessCheck(obj, member, "write to private field");
  setter ? setter.call(obj, value) : member.set(obj, value);
  return value;
};
var _e;
export class A {
}
export class B extends A {
  constructor(c) {
    _e.set(this, void 0); // ReferenceError: must call super constructor
                          // before using 'this' in derived class constructor.
    var _a;
    super();
    __privateSet(this, _e, (_a = c.d) != null ? _a : "test");
  }
  f() {
    return __privateGet(this, _e);
  }
}
_e = new WeakMap();

It seems to be a combination of using a private field, the ?? operator, and a nested value, c.d, which creates _a.