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Why overwrite "this"

In the documentation for typegoose you may have already seen the following and asked "Why do i need to overwrite this"?

class Kitten {
  // Note the following "this"
  public goEat(this: DocumentType<Kitten>, where: string) {
    // ...code to eat
  }
}

Reason for overwriting​

The reason would be that by default the type would implicitly be this: Kitten(instance), which does not give access to the document functions and would also be the wrong type and so could wrongly assume this instanceof Kitten would work, whereas that would not actually be true.

For Instance methods the implicit type would be this: Kitten(instance), whereas the correct type would be this: DocumentType<Kitten>.
For Static methods the implicit type would be this: typeof Kitten(static), whereas the correct type would be this: ReturnModelType<typeof Kitten>.

The same also applies to other places like validators or QueryHelpers where the this may need to be overwritten.

Typescript Notes​

This section has some typescript notes that are subtle and can cause confusion.

Difference between value and type​

The Type Kitten is not, infact, the class Kitten but the instance of Kitten, to actually get the static you would need to do typeof Kitten. If you would ever need to get the instance(Kitten) type of a class while you only have the input of a static(typeof Kitten), there is a Typescript Utility Type called InstanceType. (Which was also the reason why typegoose had to rename its InstanceType to DocumentType)

Example:

class Kitten {}

// T is "typeof Kitten" (static Kitten)
const T = Kitten;

// T is "Kitten" (instance of Kitten)
const T = Kitten;
// T is "Kitten" (instance of Kitten)
const T: Kitten = Kitten; // Error cannot assign "typeof Kitten" to "Kitten"
// T is "typeof Kitten" (static Kitten)
const T: typeof Kitten = Kitten;
// T is "Kitten" (instance of Kitten)
const T = new Kitten();
// T is "Kitten" (instance of Kitten)
const T: Kitten = new Kitten();
// T is "typeof Kitten" (static Kitten)
const T: typeof Kitten = new Kitten(); // Error cannot assign "Kitten" to "typeof Kitten"