76 lines
2.5 KiB
Markdown
76 lines
2.5 KiB
Markdown
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---
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title: "Async Promise Constructors"
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date: 2023-02-01T09:39:03Z
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description: Why you shouldn't use async in your new Promise()
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url: /2023/2/1/async-promise-constructors
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type: post
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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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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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-
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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) => { buf.push(data) })
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data.on('error', (err) => { reject(err) })
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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).
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