self-curry--no-memory.js

const fx = (fn, start, ...xn) => {
    const FF = class {
        constructor (...rn) {
            const ww = (...wn) => { return new FF(...rn, ...wn)}

            ww.valueOf = () => this.valueOf()
            ww.toString = () => this.toString()

            this.items = rn

            return ww
        }
        value() {return this.items.reduce(fn, start)}
        valueOf () {return this.value() - 0}
        toString () {return this.value() + ''}
    }

    return new FF(...xn)
}

self-curry.js

const fx = (fn, start, ...xn) => {
  let value;
  const fy = (...yn) => {
    value = [...xn, ...yn].reduce(fn, start);
    return fy;
  };

  fy.valueOf = () => {return value - 0};
  fy.toString = () => {return value + ''};

  return fy;
}