const Compose = (...functions) => {
    return val => {
        let i = 0,
            len = functions.length,
            fnResult = functions[i](val);

        while (++i < len) {
            fnResult = functions[i](fnResult);
        }

        return fnResult;
    };
};

const add7 = num => num + 7;
const squared = num => num * num;
const subtract13 = num => num - 13;

const add7ThenSquareThenSubtract13 = Compose(add7, squared, subtract13);

add7ThenSquareThenSubtract13(5);   // 131
add7ThenSquareThenSubtract13(0);   // 2
add7ThenSquareThenSubtract13(100); // 11436

// Let's say 'val' is 5.

// 1. Call add7(val), result is 12.
// 2. Call squared(12), result is 144
// 3. Call subtract13(144), result is 131

// subtract13( squared( add7(val)  ) ) === 131

const Compose = (...functions) => {
    return val => {
        return functions.reduce((prevVal, currFunc) => {
            return currFunc(prevVal);
        }, functions[0](val));
    };
};

const Compose = (...args) => v => args.reduce((p, c) => c(p), v)