How to get the browser viewport dimensions?

I want to provide my visitors the ability to see images in high quality, is there any way I can detect the window size?

Or better yet, the viewport size of the browser with JavaScript? See green area here:

Cross-browser @media (width) and @media (height) values 

let vw = Math.max(document.documentElement.clientWidth || 0, window.innerWidth || 0)
let vh = Math.max(document.documentElement.clientHeight || 0, window.innerHeight || 0)

window.innerWidth and window.innerHeight

• gets CSS viewport @media (width) and @media (height) which include scrollbars
• initial-scale and zoom variations may cause mobile values to wrongly scale down to what PPK calls the visual viewport and be smaller than the @media values
• zoom may cause values to be 1px off due to native rounding
• undefined in IE8-

document.documentElement.clientWidth and .clientHeight

• equals CSS viewport width minus scrollbar width
• matches @media (width) and @media (height) when there is no scrollbar
• same as jQuery(window).width() which jQuery calls the browser viewport
• available cross-browser
• inaccurate if doctype is missing

Resources

• Live outputs for various dimensions
• verge uses cross-browser viewport techniques
• actual uses matchMedia to obtain precise dimensions in any unit