textarea demo

controllers.application.js

import Ember from 'ember';

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

templates.application.hbs

<h3>Plain</h3>
<textarea id="my_textarea">hey hey</textarea/>

<br>
<h3>empty initial value + update action</h3>
{{myValue}}
<textarea value={{myValue}} oninput={{action (mut myValue) value="target.value"}}></textarea>

<br>
<h3>non-empty initial value + update action</h3>
{{testString}}
<textarea value={{testString}} oninput={{action (mut testString) value="target.value"}}></textarea>

<br>
<h3>non-empty initial value + no update action</h3>
{{testString}}
<textarea value={{testString}}></textarea>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}