@types/through2

ctor: {
    (transform?: TransformFunction, flush?: FlushCallback): Through2Constructor;
    (
        opts?: stream.DuplexOptions,
        transform?: TransformFunction,
        flush?: FlushCallback
    ): Through2Constructor;
};

obj: (transform?: TransformFunction, flush?: FlushCallback) => stream.Transform;

through2: typeof through2;

type FlushCallback = (this: stream.Transform, flushCallback: () => void) => void;

type TransformCallback = (err?: any, data?: any) => void;

type TransformFunction = (
    this: stream.Transform,
    chunk: any,
    enc: BufferEncoding,
    callback: TransformCallback
) => void;