title: How to concat url in Angular 6?
body: <p>I have common-class has commonUrl, this commonUrl i used in category.service.ts but it not concat in service.ts how to concat this commonUrl in angular 6?</p>
  
  <p>common-class.ts</p>
  
  <pre><code>export class CommonClass {
    constructor(public commonUrl : string = 'http://localhost:3000'){};
  }
  </code></pre>
  
  <p>category.service.ts</p>
  
  <pre><code>import { CommonClass } from '../classes/common-class';
  commonUrlObj : CommonClass = new CommonClass();
  
  saveNewCategory(formData){
    return this.http.post('this.commonUrlObj.commonUrl'+''+'/saveNewCategory',formData).map((res: any) =&gt; res.json());
  }
  
  getCategoryDetails(param){
    return this.http.post('this.commonUrlObj.commonUrl'+''+'getCategoryDetails',param).map((res: any) =&gt; res.json());
  }
  </code></pre>