partial&curry.js

'use strict';

const sliceArgs = function(args, beginIndex, endIndex) {
    return Array.prototype.slice.call(args, beginIndex, endIndex);
};

const _ = Symbol('placeholder');

function partial(fn) {
    let _args = sliceArgs(arguments, 1);
    
    return function() {
        let args = sliceArgs(arguments);
        args = _args.map(x => x === _ ? args.shift() : x).concat(args);

        return fn.apply(null, args);
    }
}

function curry(fn, arity) {
    isNaN(arity) && (arity = fn.length);

    return function() {
        let args = sliceArgs(arguments);
        let need = args.reduce((acc, cur) => cur === _ ? acc : --acc, arity);

        if (need > 0) {
            return curry(partial.apply(null, [fn].concat(args)), need);
        }
        
        return fn.apply(null, args);
    };
}

const say = function(name, msg) {
    console.log(`${name} says: ${msg}`)
}

const sum = function() {
    return [].reduce.call(arguments, function(acc, cur) {
        return acc + cur;
    }, 0);
};

const joenilSay = partial(say, 'Joenil');
const sayHello = partial(say, _, 'hello!');
const add = curry(sum, 3);
const addOne = add(1);
const addTen = addOne(9);

joenilSay('Good morning!')
joenilSay('Good night!')
sayHello('Joenil')
sayHello('Abby')

console.log(`
    add(1, 2, 3): ${add(1, 2, 3)}
    add(1)(2)(3): ${add(1)(2)(3)}
    add(1, 2)(3): ${add(1, 2)(3)}
    add(1)(2, 3): ${add(1)(2, 3)}

    add(_, 2, 3)(1): ${add(_, 2, 3)(1)}
    add(1)(_, 3)(2): ${add(1)(_, 3)(2)}
    add(_, _)(3)(1, 2): ${add(_, _)(3)(1, 2)}

    addOne(8, 1): ${addOne(8, 1)}
    addTen(10): ${addTen(10)}
`)