js to base64 image upload

imgChange(event, num) {
        var self = this;
        var $this = event.target; //input
        var file = $this.files[0];
        // Determine if it is an image type
        if (!/image\/\w+/.test(file.type)) {
          alert("Only pictures can be selected");
          return false;
        }
        var reader = new FileReader();
        reader.readAsDataURL(file);
        reader.onload = function (e) {
          Utils.uploadImg(this.result, function (ossUrl) {
            self.productImg['img'+num] = ossUrl;
          })
        }
      }