实现sum(1,2,3)==sum(1)(2)(3)

[LuckyWinty]

function sum(...args){
  function currySum(...rest){
    args.push(...rest)
    return currySum
  }
  currySum.toString= function(){ 
    return args.reduce((result,cur)=>{
      return result + cur
    })
  }
  currySum.toNumber= function(){ 
    return args.reduce((result,cur)=>{
      return result + cur
    })
  }
  return currySum
}

更多参考：https://github.com/Advanced-Frontend/Daily-Interview-Question/issues/134