Simple script loader

load-script.ts

const scripts = new Map<string, Promise<unknown>>();

export function loadScript(
  src: string,
  additionalScriptAttrs: Record<string, string> = {}
): Promise<unknown> {
  if (scripts.has(src)) {
    // TypeScript cannot narrow down type with `has`
    // https://github.com/microsoft/TypeScript/issues/13086
    return scripts.get(src) as Promise<unknown>;
  }

  const script = document.createElement('script');
  script.async = true;
  Object.entries(additionalScriptAttrs).forEach(([attr, value]) => {
    script.setAttribute(attr, value);
  });

  const promise = new Promise((resolve, reject) => {
    script.addEventListener('load', resolve);
    script.addEventListener('error', () =>
      reject(new Error(`Failed to load script: ${src}`))
    );
    script.src = src;
    document.head.appendChild(script);
  });

  scripts.set(src, promise);
  return promise;
}