resizeImage.js

function resizeImage(oldImg, height){
  	
    var image = new Image();
    image.src = oldImg.src;
    
    var d = image.width / image.height;
    
    var newWidth = height * d;
    var newHeight = height;
    
    oldImg.style.width = newWidth + 'px';
    oldImg.style.height = newHeight + 'px';
  
}

var imgTest = document.getElementById('testImg');
var divTest = document.getElementById('testDiv');

console.log(parseInt(divTest.style.height, 10))

imgTest.onload = function(){
    resizeImage(this, 600);
}