Fit image/content to your window so that the ratio is respected and your image is moved to the center.

fitIntoScreen.js

// FIT INTO SCREEN

// Your image or content size
var contentWidth = 1024;
var contentHeight= 768;

// The resize function, to be impletemented however you need it
function resize(){
     var w, h, ws, hs, rs, rc, r;

     // get size of the screen
     ws = window.innerWidth;
     hs = window.innerHeight;

     // calculates ratios of screen and content
     rs = ws / hs;
     rc = contentWidth / contentHeight;
     
     // we chose the correct ratio
     if(rs > rc){
          r = ws / contentWidth;
     }
     else{
          r = hs / contentHeight;
     }

     // get new width and height
     w = contentWidth * r;
     h = contentHeight * r;

     // Here it's up to you to either set the scaleX and scaleY of your content to "r" 
     // or the width and height to w and h
     // Also to move your content to the center : 
     // content.x = (ws >> 1) - (w >> 1);
     // content.y = (hs >> 1) - (h >> 1);
}