Rotates a matrix of pixels in place

rotatePicture.js

function rotatePicture(matrix) {
    if (matrix.length == 0 || matrix.length != matrix[0].length) {
        return [];
    }
    
    var n = matrix.length; 
    var center = parseInt(n/2);

    for (var i = 0; i < center; i++) {
        var start = i;
        var end = n - i - 1;
        
        for (var j = start;  j < end; j++) {
            var top = matrix[start][j];
            var left = matrix[j][end];
            var bottom = matrix[end][end - j - start];
            var right = matrix[end - j - start][start];
            
            matrix[start][j] = right;
            matrix[j][end] = top;
            matrix[end][end - j - start] = left;
            matrix[end - j - start][start] = bottom;
        }
    }
    return matrix;
}