JS constructors had a couple of bugs with strictNullChecks (which people actually do use with checkJs, see the recent post by @OliverJAsh).

1. this-assignments in a function added undefined to the type of all properties:

function A() {
  this.x = 1
}

Incorrectly had x: number | undefined not x: number.

2. Callable constructors, which have a check to see whether they were called instead of newed, incorrectly unioned undefined to the instance type. This was unneeded and annoying:

function A() {
  if (!(this instanceof A)) {
    return new A()
  }
  this.x = 1
}
var a = new A()

This incorrectly had a: { x: number } | undefined when it should have been a: { x: number }

Fixes #22414
Fixes #22641

Note that the second fix checks two things to distinguish a constructor function from a normal one:

1. Whether the function is a javascript constructor
2. Whether the function returns a type with the symbol of the constructor function itself -- that is, whether it has a statement return new A() somewhere inside function A. It doesn't check whether this statement is inside a conditional.