brainsteam.co.uk/brainsteam/content/posts/2023/02/async-promise-constructors.md

---
date: 2023-02-01 09:39:03+00:00
description: Why you shouldn't use async in your new Promise()
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tags:
- nodejs
- typescript
- javascript
- softeng
title: Async Promise Constructors
type: posts
url: /2023/2/1/async-promise-constructors
---

I ran into an interesting typescript/js problem yesterday at work.

The following code snippet was generating an error and a stack trace that was never being propagated up to the caller:

```typescript
return new Promise(async (resolve, reject) => {
  const data = await this.client.getObject(bucket, path);

  const buf: any[] = [];

  data.on("data", (data) => {
    buf.push(data);
  });

  data.on("error", (err) => {
    reject(err);
  });

  data.on("end", () => {
    resolve(JSON.parse(Buffer.concat(buf).toString()));
  });
});
```

The code basically uses [minio](https://min.io/)'s client library to get the contents of an object in an S3 storage bucket and then wraps the stream that it returns in a promise that will eventually parse the content of the S3 object into a JS object or reject if something went wrong.

Seasoned js folks may have already spotted the problem (or at least guessed it based on the blog post title).

I'ved used an `async` function inside my Promise constructor. I'm doing this so that I can make my async call to S3 but this has the added side effect of turning my promise constructer itself into a promise (promise-ception?). Promise constructors can't `await` so effectively if the `getObject` async call fails, the error is lost and nothing happens and the outer promise is never resolved or rejected.

As it turns out, [eslint has a rule for this](https://eslint.org/docs/latest/rules/no-async-promise-executor) which I had turned off.

### Solution

I could make the getObject outer function async and then do the `getObject()` call in the outer layer:

```typescript
async function someFunction() {
  const data = await this.client.getObject(bucket, path);

  return new Promise((resolve, reject) => {
    const buf: any[] = [];

    data.on("data", (data) => {
      buf.push(data);
    });
    data.on("error", (err) => {
      reject(err);
    });
    data.on("end", () => {
      resolve(JSON.parse(Buffer.concat(buf).toString()));
    });
  });
}
```

Now if the `getObject` call fails the whole `someFunction()` fails (effectively the promise that is generated by use of the await/async syntactic sugar is rejected).