This code block resizes base64 image on client side using javascript

 var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

// limit the image to 150x100 maximum size
var maxW=150;
var maxH=100;

var input = document.getElementById('input');
input.addEventListener('change', handleFiles);

function handleFiles(e) {
  var img = new Image;
  img.onload = function() {
    var iw=img.width;
    var ih=img.height;
    var scale=Math.min((maxW/iw),(maxH/ih));
    var iwScaled=iw*scale;
    var ihScaled=ih*scale;
    canvas.width=iwScaled;
    canvas.height=ihScaled;
    ctx.drawImage(img,0,0,iwScaled,ihScaled);
    alert(canvas.toDataURL());
  }
  img.src = URL.createObjectURL(e.target.files[0]);
} 

 body{ background-color: ivory; }
#canvas{border:1px solid red;} 

<input type="file" id="input"/>
<br>
<canvas id="canvas" width=300 height=300></canvas>