累加函数

toString

1. console.log 会默认 toString

function f() {}
f.toString = function() {
  return 8888
}
console.log(f) // 8888

function f() {
  return f
}
f.toString = function() {
  return 8888
}
console.log(f()) // 8888

实现 a(1)(2)(3)

function add(a) {
  function sum(b) {
    a = b ? a + b : a
    return sum
  }
  sum.toString = function() {
    return a
  }
  return sum
}

实现多参数累加

function add(a) {
  let args = [...arguments]
  function sum(b) {
    args.push(...arguments)
    return sum
  }
  sum.toString = function() {
    return args.reduce((a, b) => a + b)
  }
  return sum
}

console.log(add(1)(2))
console.log(add(1, 5)(2))