Proprotionate size changing(clientside)

Resize image to fit box.js

function resizeImgOO(el){
  function imgRatio(){
    return (el.height / el.width);
  }

  function holderRatio(){
    return (el.offsetParent.offsetHeight / el.offsetParent.offsetWidth);
  }

  function fitToContainer(){
    if(imgRatio>holderRatio){
      el.height = el.offsetParent.offsetHeight;
    } else {
      el.width = el.offsetParent.offsetWidth;
    }
  }

  this.imgRatio = imgRatio;
  this.holderRatio = holderRatio;
  this.resize = fitToContainer;
}

var img = new resizeImgOO(document.getElementById('yourImgId'));
img.resize();