TypeError: Class extends value undefined is not a function or null

I was having the same issue. It turns out I was circularly importing classes, which is apparently a limitation. (See these GitHub issues: #20361, #4149, #10712)

Note that it seems that the circular reference is also limited between files, not simply types.

See this other answer

Circular dependencies can be tricky to identify. Michael Weststrate has an interesting reading about circular dependencies and proposed a pattern to fix them.

Automated circular dependency detection.

On top of using a pattern that allows for scalability, you can make use of a super useful tool that with very little effort will identify circular dependencies for you, Madge.

Madge can be ran over .ts or .js files. I found that is useful to run it in both directories, as they might give different results due to the transpilation process.

For Typescript .ts files:

madge --circular --extensions ts <directory_path>

For Javascript .js files:

madge --circular <directory_path>

As noted in Thomas Jensen's comment above, circular references can occur not just in Types, but also in files. I encountered this same problem when I was exporting both the base and derived types from the same file. Such as:

// index.ts
export { BaseClass } from "./base";
export { DerivedClass } from "./derived";

This is an easy pitfall to fall into. Posting this here in the hopes it'll save someone else the debugging time.