I just stumbled upon the following problem :
OpenLayers is unable to retrieve a projection definition even though it has been previously registered with Proj4JS. Code below :
import proj4 from 'proj4/dist/proj4'; // Proj4JS version 2.4.4
import proj from 'ol/proj'; // OpenLayers version 4.6.5
proj4.defs('EPSG:2154', '+proj=lcc +lat_1=49 +lat_2=44 +lat_0=46.5 +lon_0=3 +x_0=700000 +y_0=6600000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs');
console.log(proj.get('EPSG:2154')); // displays null
I can not figure out anything but a version mismatch as reported in OpenLayers 3 not recognising proj4 custom projection