Async Iterator Example

async-iterator.js

main().catch(console.error);

async function main() {
  const clicks = streamify(document, "click");
  let clickCount = 0;
  for await (const event of clicks) {
    console.log("click", event, clickCount);
    if (clickCount++ > 2) {
      clicks.return();
    }
  }
  console.log("bye");
}

function streamify(element, event) {
  const pushQueue = [];
  const pullQueue = [];
  let done = false;

  function pushValue(event) {
    if (pullQueue.length !== 0) {
      pullQueue.shift()({ value: event, done: false });
    } else {
      pushQueue.push(event);
    }
  }

  function pullValue() {
    return new Promise(resolve => {
      if (pushQueue.length !== 0) {
        resolve({ value: pushQueue.shift(), done: false });
      } else {
        pullQueue.push(resolve);
      }
    });
  }

  element.addEventListener(event, pushValue);

  return {
    [Symbol.asyncIterator]() {
      return this;
    },
    next() {
      return done ? this.return() : pullValue();
    },
    return() {
      done = true;
      element.removeEventListener(event, pushValue);
      return Promise.resolve({ value: undefined, done });
    },
    throw(error) {
      done = true;
      return Promise.reject(error);
    }
  };
}

See Understand Async Iterators Without Really Trying and event-emitter-to-async-iterator.js.