19-arguments-optional.js

/*
Arguments Optional:
Create a function that sums two arguments together. If only one argument is provided, then return a function that expects
one argument and returns the sum.
For example, addTogether(2, 3) should return 5, and addTogether(2) should return a function.

Calling this returned function with a single argument will then return the sum:

var sumTwoAnd = addTogether(2);
sumTwoAnd(3) returns 5.

If either argument isn't a valid number, return undefined.

- addTogether(2, 3) should return 5.
- addTogether(23, 30) should return 53.
- addTogether(5)(7) should return 12.
- addTogether("https://www.youtube.com/watch?v=dQw4w9WgXcQ") should return undefined.
- addTogether(2, "3") should return undefined.
- addTogether(2)([3]) should return undefined.
*/
function addTogether() {
    let [arg1, arg2] = arguments;
    if (!Number.isInteger(arg1) || (arg2 !== undefined && !Number.isInteger(arg2))) {
        return undefined;
    }
    if (!arg2) {
        return function (newArg) {
            if (Number.isInteger(newArg)) {
                return arg1 + newArg;
            }
        }
    }
    return arg1 + arg2;
}

console.log(addTogether(2, 3));
console.log(addTogether(23, 30));
console.log(addTogether(5)(7));
console.log(addTogether("https://www.youtube.com/watch?v=dQw4w9WgXcQ"));
console.log(addTogether(2, "3"));
console.log(addTogether(2)([3]));