resizer.js

function resizeFormIsValid() {
    var x = +resizeX.value;
    var y = +resizeY.value;
    var side = +resizeSize.value;
    var imageWidth = currentResizer._image.naturalWidth;
    var imageHeight = currentResizer._image.naturalHeight;
    var messageWrapper = document.getElementById('upload-resize-message');

    if (x + side <= imageWidth && y + side <= imageHeight) {
      resizeFwd.disabled = false;
      resizeFwd.style.opacity = 1;
      if (messageWrapper) {
        messageWrapper.textContent = '';
      }
      // resizeMessage.classList.add('invisible');
      return true;
    }

    if (!messageWrapper) {
      messageWrapper = document.createElement('div');
      messageWrapper.id = 'upload-resize-message';
    }

    resizeFwd.disabled = true;
    messageWrapper.textContent = 'Ошибочка!';
    resizeFwd.style.opacity = .3;
    console.log(messageWrapper);
    resizeForm.appendChild(messageWrapper);
    return false;
  }
  
  
  
  resizeForm.onchange = function() {
   resizeFormIsValid();
  };