From aa23b3906c46792a90c990f45fe14795c32a629a Mon Sep 17 00:00:00 2001
From: James Ravenscroft <james.ravenscroft@filament.ai>
Date: Wed, 1 Feb 2023 09:41:08 +0000
Subject: [PATCH] added async promises post

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+---
+title: "Async Promise Constructors"
+date: 2023-02-01T09:39:03Z
+description: Why you shouldn't use async in your new Promise()
+url: /2023/2/1/async-promise-constructors
+type: post
+mp-syndicate-to:
+- https://brid.gy/publish/mastodon
+- https://brid.gy/publish/twitter
+tags:
+  - nodejs
+  - typescript
+  - javascript
+  - softeng
+  - 
+---
+
+
+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).
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