myview.js

// <canvas id="resizeCanvas" width="10000" height="10000" style="display:none"></canvas>

Template.myview.events({
  "change .image-select": function(e, t) {
    // Verify it's an image...
    
    console.time("total");
    var img = document.createElement("img");
    img.src = window.URL.createObjectURL(e.currentTarget.files[0]);
    img.onload = function () {
      var canvas = t.find("#resizeCanvas");
      canvas.width = 10000;
      canvas.height = 10000;
      var ctx = canvas.getContext("2d");

      var targetSize = 200;
      var width = img.width;
      var height = img.height;

      if (width > height) {
        height = targetSize;
        width = Math.round(width / (img.height / targetSize));
      } else {
        height = Math.round(height / (img.width / targetSize) );
        width = targetSize;
      }
      canvas.width = width;
      canvas.height = height;
      var ctx = canvas.getContext("2d");
      ctx.drawImage(img, 0, 0, width, height);
      console.log(img.width, img.height, width, height);
      console.time("export");
      canvas.toBlob(function(blob){
        var file = new File([blob], 'pic.jpg', {
          type: "image/jpeg"
          // type: "image/png"
        });
        console.timeEnd("total");
        console.timeEnd("export");
        var uploader = new Slingshot.Upload("uploadAvatar");

        uploader.send(file, function(err, url) {
          if (err) // log error and return
          // update some state
        });

      }, "image/jpeg", 0.9);
      return;
    };
    
  }
});


if (!HTMLCanvasElement.prototype.toBlob) {
 Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
  value: function (callback, type, quality) {

    var binStr = atob( this.toDataURL(type, quality).split(',')[1] ),
        len = binStr.length,
        arr = new Uint8Array(len);

    for (var i=0; i<len; i++ ) {
     arr[i] = binStr.charCodeAt(i);
    }

    callback( new Blob( [arr], {type: type || 'image/png'} ) );
  }
 });
}