sup.js

"use strict";

function overload(f1, f2) {

    if (f1.length === f2.length)
        throw new TypeError;
    else
        return function(...args) {
            let params = Array.prototype.splice.call(arguments, 0);
            if (params.length === f1.length)
                return f1(...params);
            else if (params.length === f2.length)
                return f2(...params);
            else
                throw new TypeError;
    }
}

var createVector = overload(
    (a, b) => { return { x: a,  y: b } },
    (length) => { return { x: length / 1.414,  y: length / 1.414 } }
);

console.log(createVector(3, 4));
console.log(createVector(7.07));