async.js

export async function load({id, src, onEnd}) {
  const node = await createNode(src, onEnd);
  nodes[id] = node;
  
  return {
    duration: node.duration()
  };
}

promise.js

export function load({id, src, onEnd}) {
  return createNode(src, onEnd)
    .then(node => nodes[id] = node)
    .then(node => ({
      duration: node.duration()
    }));
}