JSFiddle - React, Tailwind, and code Playground

HTML

<pre id="out"></pre>

JavaScript

const arr = [11, 22, 33];

for (const index in arr) {
  if (index === '0') {
    arr.push(99);
  }

  const value = arr[index];
  out.textContent += `${index}: ${value}\n`;
}

out.textContent += arr;