http://mathjs.org/docs/reference/functions/norm.html documents math.norm as returning a Number or a BigNumber.

However, the frobenius norm of a matrix containing complex values is a complex number (with imaginary part equal to 0).

math.norm(math.matrix([[math.complex(1, 4)]]), 'fro')
// Object { re: 4.123105625617661, im: 0 }