WriteFileCallback: sourceFiles is optional

[ivogabe]

WriteFileCallback is defined as follows:

export interface WriteFileCallback {
    (fileName: string, data: string, writeByteOrderMark: boolean, onError?: (message: string) => void, sourceFiles?: SourceFile[]): void;
}

I was wondering why sourceFiles marked as optional. As of version 3.6, we're using this in gulp-typescript. This works fine, but ivogabe/gulp-typescript#499 describes that sourceFiles is undefined in some special case. So, when is sourceFiles undefined, and is there some way to force it being set?

[ivogabe]

@vladima Any ideas?

[mhegazy]

That is a bug int he declaration, soruceFiles should always be passed, it can be a single file or all files in the program.

[DickvdBrink]

@mhegazy, this is a bit difficult to change because onError is also optional. So making to make sourceFiles mandatory it is necessary to change the order of the parameters which is a bit difficult in a public API

[ivogabe]

Why is onError optional? It could also be marked as | undefined instead, or is this parameter also always provided and not optional?