so thats how my normals in a test object looks like
json (wrong):
"normals": [0,-0.923856,-0.382672,0,-1,0,0,0,-1,0,-0.382672,-0.923856],
obj (good):
vn 0.000100 1.000000 0.000000
vn 0.000000 1.000000 0.000000
vn 0.000000 0.000000 1.000000
so it looks like i would call .computeVertexNormals() on the geometry before exporting to json