How to calculate the azimuth of two vectors

edge_vec = edge_start_pt.vector_to edge_end_pt
edge_vec.reverse! if edge_vec.y > 0

In my code my plane is at xz axis

From the above code i decode

edge_vec = edge_start_pt.vector_to edge_end_pt

This to :

edge_vec = new THREE.Vector3().subVectors(edge_start_pt, edge_end_pt).normalize();

what is the equivalent for this in three js

edge_vec.reverse! if edge_vec.y > 0

I am using the same about formula but for this x axis. Writting you because your calculation are helpfull.

Here i am trying to find the skew of an line with respect to x axis. Please suggest me if you have any better idea