普通

function add(num1) {
  let res = num1;
  function sum(num2) {
    if (num2) {
      res += num2;
      return sum;
    }
  }
  sum.toString = function () {
    return res;
  };
  sum.done = function () {
    return res;
  };

  return sum;
}

console.log(add(1)(2));
console.log(add(1)(2).done());

科里化

科里化实现无限，add(1,2)(3,4,5)(6)

function add(...args1) {
  let res = [...args1];

  function sum(...args2) {
    if ([...args2].length > 0) {
      res = [...args1, ...args2];
      return sum;
    }
  }
  sum.toString = function () {
    return res.reduce((a, b) => a + b);
  };
  sum.done = function () {
    return res.reduce((a, b) => a + b);
  };

  return sum;
}

console.log(add(1)(2, 3, 4, 5));