reorder.js

const origin = '(a,(b,(c,(d,(e,(f))))))'

function reorder(str) {
  let matches = str.match(/^\((\w),(.*)\)$/)
  if (matches) {
    const [left, right] = reorder(matches[2])
    return [`${left},(${matches[1]}`, `)${right}`]
  } else {
    let matches = str.match(/^\((\w)\)$/)
    return [`(${matches[1]}`, ')']
  }
}

let [left, right] = reorder(origin)
console.log(`${left}${right}`)