Based off of Nicolas's answer - https://dev.to/minimumviableperson/comment/c0b9

add numbers by currying.js

// refer to https://dev.to/minimumviableperson/comment/c0b9
// by @minimumviableperson

const double = x => x*2;
const addOne = x => x+1;

const addFn = (...args) => {
    let count = 0;

    const map = {
        'function': (acc, arg) => arg(acc),
        'number': (acc, arg) => arg + acc,
    }
    const apply = (acc, arg) => map[typeof arg](acc, arg) || acc;
    const applyArgs = args => args.reduce(apply, count);

    const adder = (...args) => args[0] === undefined
        ? count
        : (count = applyArgs(args), adder);

    adder.valueOf = () => count;

    return adder(...args);
}


// const addFn = (...args) => {
//     let count = 0;

//     const applyArgs = args => args.reduce((acc, arg) => {
//         switch (typeof arg) {
//             case 'function': return arg(acc);
//             case 'number': return arg + acc;
//             default: return acc;
//         }
//     }, count);

//     const adder = (...args) => args[0] === undefined
//         ? count
//         : (count = applyArgs(args), adder);

//    adder.valueOf = () => count;

//     return adder(...args);
// }




// const addFn = (...args) => {
//     let count = 0;

//     const applyArgs = args => args.reduce((acc, arg) => {
//         switch (typeof arg) {
//             case 'function': return arg(acc);
//             case 'number': return arg + acc;
//             default: return acc; 
//         }
//     }, count);

//     const adder = (...args) => {
//         if (args[0] === undefined) return count;

//         count = applyArgs(args);
//         return adder;
//     }

//     adder.valueOf = () => count;


//     return adder(...args);
// }

A deliberate practice to understand currying and spread/rest operators.