title: Multiple Api calling serialy in angular 6
body: <p>I am having 20 APIs. My goal is to create a page. When the page will get loaded i want All my APIs to be called and load the data and store it in many array type variables. But if I am calling all the APIs on initialisation its taking too much time to load the page. 
  So now i want to call the APIs one by one automatically on page initialisation. Can you please give me some idea how to make these APIs call  one by one serialy.
  If i am using if clause i.e <em>if(respose)</em> at the end of each API promises the code seems too lengthy.</p>