function sumUpDiagonals(matrix) {

    try {
        const len = matrix.length;

        matrix.forEach((subArray) => {
            if(subArray.length !== len) {
                throw new SyntaxError('не вкадратная матрица');
            }
        });

        return matrix
            .map((subArray, firstElIndex) => {
                const secondElIndex = subArray.length - 1 - firstElIndex,
                    firstEl = subArray[firstElIndex],
                    secondEl = subArray[secondElIndex];

                const notNum = (element) => typeof element !== 'number' || isNaN(element);

                if (notNum(firstEl) || notNum(secondEl)) {
                    throw new SyntaxError('Input data is not a number.')
                }
                return firstEl === secondEl ? (firstEl * 2) : (firstEl + secondEl);
            })
            .reduce((prevElement, nextElement) => prevElement + nextElement);

    } catch (error) {
        return error;
    }
}

let matrixExample = [
    [ 1, 2, 3, 4,],
    [ 4, 5, 6, 5 ],
    [ 7, 8, 9, 7 ],
    [ 7, 8, 9, 7 ]
];

alert(sumUpDiagonals(matrixExample));