code: function lineToLineSourceMap(source, filename) {
    // The first line mapping in our package is the base64vlq code for zeros (A).
    const firstLine = 'AAAA;';
  
    // Most other lines in our mappings are all zeros (for module, column etc)
    // except for the lineno mapping: curLineno - prevLineno = 1; Which is C.
    const line = 'AACA;';
  
    return {
      version: 3,
      sources: [filename],
      mappings: firstLine + Array(countLines(source)).join(line),
    };
  }
docs: fastest implementation