柯里化

const join = (a, b, c) => {
  return `${a}_${b}_${c}`;
};

const curriedJoin = curry(join);

curriedJoin(1, 2, 3); // '1_2_3'

curriedJoin(1)(2, 3); // '1_2_3'

curriedJoin(1, 2)(3); // '1_2_3'

function curry(fn) {
  return function curried(...args) {
    // if number of arguments match
    if (args.length >= fn.length) {
      return fn.call(this, ...args);
    }
    return function (...missingArgs) {
      return curried.call(this, ...args, ...missingArgs);
    };
  };
}