The Problem

Currently, MathNode is defined as a union of all of the existing subclasses of Node i.e.

  type MathNode = MathNo
    | AccessorNode
    | ArrayNode
    | AssignmentNode
    | BlockNode
   ...

This has a few consequences:

1. Every time a node new type is added in MathJS you also have to remember to add that type to the MathNode union
2. It's impossible (or at least as far as I can tell) to add a custom subclass of Node when using mathjs while having that node conform to the MathNode type so that TS is happy

The solution

By simply aliasing MathNode to MathNodeCommon (which all existing node types extend), typescript automatically infers that any subclasses of Node are valid MathNode types.

I did some digging and couldn't quite figure out why the MathNodeCommon type needed to exist in the first place and why it couldn't just be called MathNode, but it is what it is and I guess we can't change that now without a lot of breaking changes.