New Twiddle

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  tagName: 'a',
  attributeBindings: ['download', 'href', 'target'],

  download: Ember.computed('isPDF', function() {
    if (this.get('isPDF')) {
      return null;
    } else {
      return 'filename to download';
    }
  }),

  // Open PDFs in a new tab
  target: Ember.computed('isPDF', function() {
    if (this.get('isPDF')) {
      return null;
    } else {
      return '_self';
    }
  }),

  isPDF: Ember.computed('href', function() {
    const src = this.get('href');
    return src && src.match(/pdf$/);
  })
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  filename: 'something.png',
  
  actions: {
   	changeToPDF() {
      this.set('filename', 'something.pdf');
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

<ul>
  <li>
    {{#my-component href='image.png'}}Download{{/my-component}}
  </li>
  <li>
    {{#my-component href='document.pdf'}}Open{{/my-component}}
  </li>
  <li>
  	{{#my-component href=filename}}My download attribute is not removed when re-rendered{{/my-component}}
  </li>
</ul>

<p>
If you check the download attributes you'll see that they are set with the correct logic in the component. However, when the bottom link is re-rendered because the href has changed, the download attribute is not removed from the DOM, it is still there with the value of null.
</p>

<button {{action 'changeToPDF'}}>Change to PDF</button>

templates.components.my-component.hbs

{{yield}}

twiddle.json

{
  "version": "0.8.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.4.5",
    "ember-data": "2.5.1",
    "ember-template-compiler": "2.4.5"
  }
}