How to set default class property value in TypeScript declaration file?

Question

f.e., I have

declare class Foo extends Bar {
    foo: number
}

How do I declare that foo has a default value (or initial value) of, say, 60.

I tried

declare class Foo extends Bar {
    foo: number = 60
}

but I get an error like

4     foo: number = 60
                    ~~

path/to/something.js/Foo.d.ts/(4,28): error TS1039: Initializers are not allowed in ambient contexts.

Answer 1

Try removing declare from your class definition. By using declare it will define a class type. The type is only defined, and shouldn't have an implementation.

class Foo extends Bar {
    foo: number = 60
}

Answer 2

Your program attempts to perform two mutually contradictory tasks.

1. It tries to declare that a class exists but is actually implemented elsewhere/otherwise.
2. It tries to define that implementation.

You need to determine which of these tasks you wish to perform and adjust your program accordingly by removing either the initializer or the declare modifier.