requestRange(begin, end) {
    end = Math.min(end, this.length);

    const beginChunk = this.getBeginChunk(begin);
    const endChunk = this.getEndChunk(end);

    const chunks = [];
    for (let chunk = beginChunk; chunk < endChunk; ++chunk) {
      chunks.push(chunk);
    }
    return this._requestChunks(chunks);
  }
Loads any chunks in the requested range that are not yet loaded.