Load/execute JavaScript asynchronously

load.js

function loadJs(url){
  return new Promise( (resolve, reject) => {
    if (document.querySelector(`head > script[src="${src}"]`) !== null) return resolve()
    const script = document.createElement("script")
    script.src = url
    script.onload = resolve
    script.onerror = reject
    document.head.appendChild(script)
  });
}

Usage:

try { 
 await loadJs("https://.../script.js");
} catch( error ) {
  console.log(error);
}