Resize an element when the visitor create(or remove) a new line on a text field (like a textarea)

AutoSize.js

function AutoSize(textarea) {
    textarea.className += ' js-autosize';

    function resize() {
      textarea.style.height = 'auto';

      var height = textarea.scrollHeight;

      if (window.getComputedStyle) {
        var styles = window.getComputedStyle(textarea);

        height -= parseInt(styles['padding-top']) + parseInt(styles['padding-bottom']);
      }

      textarea.style.height = height + 'px';
    }

    if (!textarea.oninput) {
      textarea.oninput = function () {
        resize();
      };
    } else {
      textarea.onkeyup = function () {
        resize();
      };
    }

    return { 'textarea': textarea, 'resize' : resize }
  }