From #12212 (comment) @joeywatts:

class A {
    property = 'some string';
}
class B extends A {
    property;
}
const instanceB = new B();
// "some string" or "undefined" ?
console.log(instanceB.property);

It appears from the current definition at https://github.com/tc39/proposal-class-fields that this code will print undefined.

If the proposal advances further with these semantics, we'll need to figure out a mitigation strategy, preferably sooner rather than later.

One option is to make the code as written a type error for ~2 versions and force people to write an explicit = undefined initializer, then remove the error.

Edit [@sandersn]:

Here is the current plan, together with what has happened so far.

TypeScript 3.6

• Allow accessors in ambient class declarations #32787: allow accessors like declare class C { get p(): number; } (and set as well)
• No changes to .js emit yet
• No changes to .d.ts emit yet

TypeScript 3.7

• Always emit accessors in .d.ts files #33470: emit get/set accessors in declaration files
• Disallow property/accessor overrides #33401: accessors may not override properties and vice versa.
• Disallow uninitialised property overrides #33423: uninitialised properties may not override properties.
  • Introduce new syntax, class C extends B { declare x: number }
  • Introduce a codefix that inserts declare where needed.
  • This declaration is erased, like the original declaration in old versions of TS
• Add useDefineForClassFields flag for Set -> Define property declaration #33509 Introduce a new flag, useDefineForClassFields.
  • When "useDefineForClassFields": false:
    • Initialized fields in class bodies are emitted as constructor assignments (even when targeting ESNext).
    • Uninitialized fields in class bodies are erased.
    • The preceding errors are silenced.
  • When "useDefineForClassFields": true:
    • Fields in class bodies are emitted as-is for ESNext.
    • Fields in class bodies are emitted as Object.defineProperty assignments in the constructor otherwise.
  • In 3.7, "useDefineForClassFields" defaults to false
  • Declaration emit is the same regardless of the flag's value.

TypeScript 3.8

(or, the first version after class fields reaches Stage 4)

• "useDefineForClassFields" defaults to true when targetting ESNext.
• issue new errors as suggestions with codefixes, even when "useDefineForClassFields": false.