title: Angular 5 disable and enable checkbox based on condition
body: <p>I'm trying to implement a validation in which i want to disable the button if a specific value entered by user matches the value returned from a service, below is my code:</p>
  
  <p>In the component, i call the service which returns the usernames like below, here is the console log for (UserNames):</p>
  
  <pre><code>0:{Name: "John", userId: "23432"}
  1:{Name: "Smith", userId: "12323"}
  2:{Name: "Alan", userId: "5223"}
  3:{Name: "Jenny", userId: "124"}
  </code></pre>
  
  <p>in the template, i use NgFor to iterate over the usernames like below</p>
  
  <pre><code>    &lt;div *ngFor="let id of UserNames let i = index;"&gt;  
      &lt;input type="radio" name="radio" [checked]="UserNames.userid" (click)="Submit(UserInput)"&gt; &lt;span&gt;Submit&lt;/span&gt;
    &lt;/div&gt;
  </code></pre>
  
  <p>What i want to achieve is if i enter 23432 the button should disabled because the service already returns userId with this value, unless a new user id entered the button should be enabled.</p>