Breakpoints JS

Following along the line drawn in this post I’ve put together a JS file to help me with those same break points, and any other break point set really, to shape behaviors accordingly.
That’s not a wide usage case but it’s a possibility added and not removed.
I’ve put the file in a gist and it allows JS calls like

if ( window.size.isXSmall() ) { ... } 

or

if ( window.size.isRem( 34 ) ) { ... } 

right now it uses JS alone and jQuery is not a requirement.
The use of the window.Size.method invocation signature is not mandatory, although very eloquent, and any method can be called like

size.isRem( ... ); 

I’ve used it to move content in mobile and desktop layouts (to avoid many presentational classes) and to modify elements on the page taking options into account (a thing that can’t be done using CSS alone without many more presentational classes).
The window.size object is there as the file will instantiate it when included in the page, removing it is as easy as commenting the line out and renaming is a question of, well, renaming it like

var MyBreakpoints = new window.Size( myBreakpoints ); 

I appreciate your input