ryasmi

var triangular = function (value) {
    var abs = Math.abs(value);
    return ((abs / 2) * (abs + 1)) * (abs / value) || 0;
};

// Testing code.
var testTriangular = function () {
    var testTriangularValue = function (arg, value, id) {
        console.log(triangular(arg) === value ? "Test " + id + " passed." : "Test " + id + " failed.");
    };