export canvas after shrinking to a size

toDataURLwithLimit.js

/**
  * @param  {!HTMLCanvasElement} canvas
  * @param  {!Number|('b'|'kb'|'mb'|'gb')} targetSize
  * @param  {?{type: ('image/png'|'image/jpeg'), jpegQuality: Number, shrinkPercentOfOriginal: Number}} options
  */
 function toDataURLwithLimit(canvas, targetSize, options){
	options = options || {};
	options.type = options.type || 'image/jpeg';
	options.jpegQuality = options.jpegQuality || .75;
	options.shrinkPercentOfOriginal = options.shrinkPercentOfOriginal || 10;
	var decrement = options.shrinkPercentOfOriginal / 100;
	 
   //parse size
   if(!targetSize || !canvas){ throw new Error('Some arguments are wrong or missing'); }
   var unit = '';
   if(!isNaN(targetSize)){
	   unit = 'size';
   }else{
	   var unit = (targetSize.match(/b|kb|mb|gb/gi) || [false])[0];
	   if(!unit){ throw new Error('Unknown unit: Use "b", "kb", "mb" or "gb".'); }
	   var tSize = (targetSize.match(/\d+/gi) || [false])[0];
	   if(!tSize){ throw new Error('Unknown size: target size has to have a number'); }
	   
   }
   switch(unit){
	   case 'b':
	   tSize = (tSize * (4 / 3));
	   break;
	   case 'kb':
		   tSize = (tSize * (4 / 3)) * 1000;
	   break;
	   case 'mb':
		   tSize = (tSize * (4 / 3)) * 1000000;
	   break;
	   case 'gb':
		   tSize = (tSize * (4 / 3)) * 1000000000;
	   break;
   }
   function sizeO(s){
	   return {
		   size: s,
		   b: s * (3 / 4),
		   kb: s * (3 / 4) / 1000,
		   mb: s * (3 / 4) / 1000000,
		   gb: s * (3 / 4) / 1000000000
	   }
   }
   tSize = sizeO(tSize);

   function act(rto){
	   rto -= decrement;
	   var src = resize(rto);
	   var sizes = sizeO(src.length);
	   
	   if (sizes[unit] < tSize[unit] || rto == decrement){
		   return src;
	   }else{
		   return act(rto);
	   }
   }
   function resize(r){
		   var rcvs = document.createElement('canvas');
		   rctx = rcvs.getContext('2d');
		   
		   rcvs.width = canvas.width * r;
		   rcvs.height = canvas.height * r;
		   
		   rctx.drawImage(canvas, 0, 0, rcvs.width, rcvs.height);
		   return rcvs.toDataURL(options.type, options.jpegQuality);
   }
   
   return act(1 + decrement);
}