Some people prefers using a full bundle Proj4js (https://github.com/DanielJDufour/proj4-fully-loaded). For server side, I like the approach but for client side, it bothers me to get the full bundle as I prefer loading on the fly the projection(s)
I made a short sample with two approaches (see comments in index.js). Code depends from availability from http://epsg.io and related APIs
git clone https://gist.github.com/f5a6a1b8d5a2b7c3f6c8f8d3745c535c load-proj4js-on-the-fly
npm i
node index.js