[SuperZen]

схематично как-то так:
index.html

<script>
  const Process = function () {
    var count = 1
    var bs = []
    return {
      req1: async () => {
        var a = await fetch('http://localhost:2999/get1.php')
        var a_r = await a.text()
        if (parseInt(a_r) === 1) {
          var intervalId = setInterval(async () => {
            count++
            var b = await fetch('http://localhost:2999/get2.php')
            var b_r = await b.text()
            bs.push(b_r)
            if (count > 5) {
              console.log(count, bs)
              clearInterval(intervalId)
            }
          }, 500)
        }
      }
    }
  }

  const instance = new Process();
  instance.req1()

</script>

get1.php

<?php
echo '1';

get2.php

<?php
echo '2';