In JS, property in derived class has wrong type with strict null checks on

[sandersn]

With strict null checks and checkJs on:

class Base {
    constructor() {
        this.built = false
    }
}
class Derived extends Base {
    init() {
        this.built = true
    }
    build() {
        if (this.built) {
            console.log('Nothing to do')
        }
        else {
            console.log('Starting build...')
        }
    }
}

Expected behavior:
No error and Derived.built: boolean.

Actual behavior:
Derived.built: boolean | undefined, causing an error on class Derived: "Class Derived incorrectly extends class Base. Types of property 'built' are incompatible: 'boolean | undefined' =/=> 'boolean'.

Found in webpack, where Base=Module and Derived=NormalModule (and many others).