autoexpand-textarea

components.my-component.js

import Ember from 'ember';

export default Ember.TextArea.extend({
	rows: 2,
  keyDown(ev) {
		let keyCode = ev.keyCode;
    if (keyCode === 8 || keyCode === 45) {
			this.$().attr('rows', 2);
    }
  },
  
  didInsertElement() {
    this._super(...arguments);

    let $this = this.$();
    let rows = this.get('rows') || '2';
    this.initialPaddingTop = parseInt($this.css('padding-top'), 10);
    this.initialPaddingBottom = parseInt($this.css('padding-bottom'), 10);
    this.initialHeightPerRow = ( this.element.clientHeight - this.initialPaddingTop - this.initialPaddingBottom ) / Number(rows);

    this.input();
  },

  input() {
    let rows = ((this.element.scrollHeight - this.initialPaddingTop - this.initialPaddingBottom) / this.initialHeightPerRow).toFixed(0);
    this.$().attr('rows', rows);
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{my-component value=value}}
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.8.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.5.1",
    "ember-data": "2.5.2",
    "ember-template-compiler": "2.5.1"
  }
}