Codebase Compiler Types

Type Hierarchy

Root class: Type

How are properties stored and found on a type?

checkPropertyAccessExpressionOrQualifiedName

getDeclaredTypeOfSymbol vs getTypeOfSymbol

The problem is that symbols can have both types and values associated with them:

type A = number
const A = "do not do this"

And the compiler needs a way to get the type of both the type and the const. So it uses getDeclaredTypeOfSymbol for types and getTypeOfSymbol[AtLocation] for values:

getDeclaredTypeOfSymbol(A) == number
getTypeOfSymbol(A) == string

Confusingly, classes (and enums and aliases) declare both a type and a value, so, a tiny bit arbitrarily, the instance side is the type and the static side is the value:

class C {
   m() { }
   static s() { }
}
getTypeOfSymbol() == { new(): C, s(): void } == typeof C
getDeclaredTypeOfSymbol() == { m(): void } == C

This kind of makes sense when you think about that C actually does when executed: it defines a value that is constructable. This leads to the "deconstructed class" pattern used in tricky situations, for example:

interface C {
  m(): void
}
var C: {
  new(): C
  s: void
}

Again, it's a tiny bit arbitrary to choose the static side as the value, since ultimately you get a value from calling new C() too. But the deconstructed class pattern shows that you can get away with writing just a type for the instance side, whereas you must write a value for the static side.

[1]: <src/compiler/checker.ts - function checkPropertyAccessExpressionOrQualifiedName(