title: How to make a string constant in angular 4?
body: <p>In my service, I am using a <code>http</code> post. I want to set the URL as a constant. </p>
  
  <pre><code>return this.http.get(this.config.API_URL+'users', options).map(res=&gt;res.json());
  </code></pre>
  
  <p>I tried with a service:</p>
  
  <pre><code>import {Injectable} from '@angular/core';
  
  @Injectable()
  export class AppService {
    API_URL :String;
  
    constructor() {
      this.API_URL = 'some url';
    }
  }
  </code></pre>
  
  <p>Is there any other method to make a constant value in Angular4 ?</p>