不确定参数的处理

ES5利用arguments

// ES5
function sum () {
  let num = 0
  Array.prototype.forEach.call(arguments, function (v) {
    num += v * 1
  })
  return num
}
console.log(sum(1, 2, 3, 4, 5))
15

ES6不推荐arguments，用扩展符

// ES6
let sum = (...arr) => {
  let num = 0
  arr.forEach(v => {
    num += v * 1
  })
  return num
}

console.log(sum(1, 2, 3, 4, 5))
15