generated types incompatible with exactOptionalPropertyTypes

[ritschwumm]

the generated types contain some contradictions which typescript does not detect due to the strict flag in the tsconfig.json being set to false.

this is a problem when a consuming project tries to enable exactOptionalPropertyTypes: now typescript considers these contradictions to be hard errors and refuses to compile.

one example are VectorTileSource and the interface ISource it implements:

ISource says "the prepare property does not have to exist, but if it exists, it has to be a function."

interface ISource extends Evented<SourceEvents> {
    ...
    readonly prepare?: () => void;
    ...
}

On the other hand VectorTileSource says "the property prepage must exists, and it has to be a value of type undefined.

export declare class VectorTileSource extends Evented<SourceEvents> implements ISource {
    ...
    prepare: undefined;
    ....
}

i'm not sure why VectorTileSource requires a prepare property to exist - if this is not strictly necessary, just removing the prepare property would solve the issue. May be confusing for a user who sets prepare and it unexpectedly does not do anything, though.

another way to solve this would be to change ISource do use readonly prepare?: () => void|undefined; which is a bit unsatisfying because now there are two different ways to express that prepare is not a function which should be called.

in general i think a typescript library should strive for the most strict typescript variant as not to limit how strict a consuming project wants to be configured.