Howto get bootstrap grid row or grid container width in pixels by javascript

Howto get bootstrap grid row or grid container width in pixels by javascript.

I’m using aurelia for javascript but code can be in standard js (please no jquery).

I tried to find this in boostrap website but i’m still confused because there are several dimensions for width: col-xs, colmd… but in fact we are just using 4 dimensions: col-3, col-6, col-9, col-12 and nothing more, so basically i need to get that full width in order to translate pixels to col-x again to match resizing from an html element.