回调

function loadScript(src, callback) {
  let script = document.createElement("script");
  script.src = src;

  script.onload = () => callback(null, script);
  script.onerror = () => callback(new Error(`Script load error for ${src}`));

  document.head.append(script);
}

回调地狱

loadScript("1.js", function (error, script) {
  if (error) {
    handleError(error);
  } else {
    // ...
    loadScript("2.js", function (error, script) {
      if (error) {
        handleError(error);
      } else {
        // ...
        loadScript("3.js", function (error, script) {
          if (error) {
            handleError(error);
          } else {
            // ...加载完所有脚本后继续 (*)
          }
        });
      }
    });
  }
});