实现 sum(1,2,3)==sum(1)(2)(3)

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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
}