node_modules/cli-progress/lib/format-bar.js

// format bar
module.exports = function formatBar(progress, options){
    // calculate barsize
    const completeSize = Math.round(progress*options.barsize);
    const incompleteSize = options.barsize-completeSize;

   // generate bar string by stripping the pre-rendered strings
   return   options.barCompleteString.substr(0, completeSize) +
            options.barGlue +
            options.barIncompleteString.substr(0, incompleteSize);
}