mathjs/src/function/arithmetic/mod.js

/**
 * Calculates the modulus, the remainder of an integer division.
 *
 *     x % y
 *     mod(x, y)
 *
 * For matrices, the function is evaluated element wise.
 *
 * @param  {Number | Complex | Array | Matrix} x
 * @param  {Number | Complex | Array | Matrix} y
 * @return {Number | Array | Matrix} res
 */
math.mod = function mod(x, y) {
    if (arguments.length != 2) {
        throw newArgumentsError('mod', arguments.length, 2);
    }

    // see http://functions.wolfram.com/IntegerFunctions/Mod/

    if (isNumber(x) && isNumber(y)) {
        // number % number
        return _mod(x, y);
    }

    // TODO: implement mod for complex values

    if (x instanceof Array || x instanceof Matrix ||
        y instanceof Array || y instanceof Matrix) {
        return util.map2(x, y, math.mod);
    }

    if (x.valueOf() !== x || y.valueOf() !== y) {
        // fallback on the objects primitive values
        return math.mod(x.valueOf(), y.valueOf());
    }

    throw newUnsupportedTypeError('mod', x, y);
};

/**
 * Calculate the modulus of two numbers
 * @param {Number} x
 * @param {Number} y
 * @returns {number} res
 * @private
 */
function _mod(x, y) {
    if (y > 0) {
        if (x > 0) {
            return x % y;
        }
        else if (x == 0) {
            return 0;
        }
        else { // x < 0
            return x - y * Math.floor(x / y);
        }
    }
    else if (y == 0) {
        return x;
    }
    else { // y < 0
        // TODO: implement mod for a negative divisor
        throw new Error('Cannot calculate mod for a negative divisor');
    }
}