title: Angular make multiple http request but wait for each one to finish before making a new one
body: <p>In angular app I have an array of literal objects containing a url property.
  I need to make a http request for each of these object, but one after another.</p>
  <p>example:</p>
  <pre><code>let arr = [
      {url: 'http://example.com/url1'},
      {url: 'http://example.com/url2'},
      {url: 'http://example.com/url3'}
  ]
  </code></pre>
  <p>(that is an example, they may be more of objects, and I don't know how many)</p>
  <p>Now, I want to make a request to first url and when we have a response from it (or error, doesn't matter) THEN I want to make a request to second etc. Any idea how to efficiently implement that?</p>
  <p>I don't want to make these request at single time - each one should be made only after previous was successful or failure.</p>