mapping degree of $f: X \rightarrow Y$ with $\dim X \neq \dim Y$

Question

I am writing about the mapping degree (also called Brouwer degree or topological degree). When calculating the degree for a function $f$, one has to use the determinant of the Jacobian of $f$. This only makes sense, if the Jacobian is a square matrix. However, I read that the mapping degree is also defined for functions $f: X \rightarrow Y$ with $\dim X \neq \dim Y$ (as opposed to the fixed point index). I haven't been able to find a construction of the mapping degree for this case. Does anyone know how to calculate the mapping degree for $\dim X \neq \dim Y$ or any literature that could help?