title: trigger a synchronous call inside loop wait till two api calls get success then next iteration need to starts in angular 6
body: <p>Tried with below code not wait for post call success jumping to next iteration before response comes.
  Requirement:Need to have next iteration after the success of two api(POST/PATCH) calls</p>
  
  <pre><code> for (item of data) {
    A(item)
  
  }
  A(value) {
    const resp = this.post(url, {
        'rationale': value['rationale']
      })
      .mergeMap(tempObj =&gt; {
        value['detail'] = tempObj['id']
        return this.patch(url, value['extid'], value)
      })
      .subscribe()
  }
  </code></pre>