76 lines
2.5 KiB
Markdown
76 lines
2.5 KiB
Markdown
---
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date: 2023-02-01 09:39:03+00:00
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description: Why you shouldn't use async in your new Promise()
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mp-syndicate-to:
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- https://brid.gy/publish/mastodon
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- https://brid.gy/publish/twitter
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post_meta:
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- date
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preview: /social/2dc42dc49c5694f51e252826163f1b799e811664bd8d7defc90ce9e01af09aad.png
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tags:
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- nodejs
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- typescript
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- javascript
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- softeng
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title: Async Promise Constructors
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type: posts
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url: /2023/2/1/async-promise-constructors
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---
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I ran into an interesting typescript/js problem yesterday at work.
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The following code snippet was generating an error and a stack trace that was never being propagated up to the caller:
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```typescript
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return new Promise(async (resolve, reject) => {
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const data = await this.client.getObject(bucket, path);
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const buf: any[] = [];
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data.on("data", (data) => {
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buf.push(data);
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});
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data.on("error", (err) => {
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reject(err);
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});
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data.on("end", () => {
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resolve(JSON.parse(Buffer.concat(buf).toString()));
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});
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});
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```
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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.
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Seasoned js folks may have already spotted the problem (or at least guessed it based on the blog post title).
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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.
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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.
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### Solution
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I could make the getObject outer function async and then do the `getObject()` call in the outer layer:
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```typescript
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async function someFunction() {
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const data = await this.client.getObject(bucket, path);
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return new Promise((resolve, reject) => {
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const buf: any[] = [];
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data.on("data", (data) => {
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buf.push(data);
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});
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data.on("error", (err) => {
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reject(err);
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});
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data.on("end", () => {
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resolve(JSON.parse(Buffer.concat(buf).toString()));
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});
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});
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}
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```
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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). |