How to wait a function to end in JavaScript?

Question

the JavaScript code below shows that the line "b" works before line "a" because line a needs some time to finish

I want line "b" to work after line "a"

var time = Math.floor(Math.random() * 11) * 1000;

function tt(){ // example function with unknown time to end
  var tts = setInterval(function() {
     document.getElementById("a").innerHTML = "a";
     clearInterval(tts);
  }, time);
}
//----------------
tt(); //line a

document.getElementById("b").innerHTML = "b"; //line b

<p id="a"></p>
<p id="b"></p>