This cast is unfortunate because the EncodedSourceMap in trace-mapping

export interface SourceMapV3 {
    file?: string | null;
    names: string[];
    sourceRoot?: string;
    sources: (string | null)[];
    sourcesContent?: (string | null)[];
    version: 3;
}
export interface EncodedSourceMap extends SourceMapV3 {
    mappings: string;
}

is slightly different to the one in gen-mapping.

export interface SourceMapV3 {
    file?: string | null;
    names: readonly string[];
    sourceRoot?: string;
    sources: readonly (string | null)[];
    sourcesContent?: readonly (string | null)[];
    version: 3;
}
export interface EncodedSourceMap extends SourceMapV3 {
    mappings: string;
}

@jridgewell Maybe you want to align them

We shouldn't use declare here, since the constructor then sets these property (and we do not want to walk the prototype checking if they are already there)

I mark them as declare because currently class properties are transpiled to initialization assignments in constructor as we are targeting Node.js 6: https://unpkg.com/@babel/core@7.22.9/lib/transformation/plugin-pass.js, this change will get rid of such extra assignments.

But since we will ship native class properties in Babel 8, I am happy to revert that.

I wish I could get rid of the typing cast here, the typeof check here can not rule out {} completely as it could be Function object.

Suggestions are welcome.

@ehoogeveen-medweb My bad. I was meant for object as I typed value as an union of object and a specific function:

but for some reason the typeof check does not rule out the object completely.