await-of | await wrapper for easier errors handling without try-catch | Reactive Programming library

for..in calls EnumerateObjectProperties, which does:

Return an Iterator object (26.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

All object properties are either strings or symbols. Even properties on arrays which can be set and retrieved as if they were numbers are interpreted as strings (and retrieved as strings when retrieved via for..in or other property enumeration methods like Object.keys or Object.getOwnPropertyNames). You can also see [[OwnPropertyKeys]](), which says:

The Type of each element of the returned List is either String or Symbol.

For example, with Array.prototype.push, you can see:

You need to put the token * in from of you method:

After digging a little further into the problem, it appears it is due to multiple issues here.

The async iterator can't rely on its context (e.g. this.i) to access properties that are not part of the AsyncIterator interface. Doing so will not compile in the current version of TypeScript 3.6 (even if it works fine in JavaScript), so to work around that we are left with:

If you scroll down till the specification section, you'll see that this is in the "draft" stage, so it is not yet fully specified, and the browser support is also not quite there. Your environment probably doesnt support it yet.

Is asynchronous iteration (for-await-of loop) faster than using Promise.all?

No. Both the loop and Promise.all will finish when the last promise resolves, which will be roughly at the same time. If the last promise that resolves is the first promise in the array, then Promise.all finishes immeadiately while the loop still has to iterate the other elements which might cause a small overhead but that should not matter. The only situation were it really matters is:

If one of the promises gets rejected, Promise.all exits immeadiately, while the loop has to reach that promise.

I'm wondering if asynchronous iteration starts looping before all the pages are finished fetching.

Yes you could get the first results a bit earlier, but all results will be available at the same time. Using the for loop would make sense if you want to show the data to the user, as he can start reading while the rest is still fetching, if you however want to accumulate the data it makes sense to await them all as it simplifies the iteration logic.

Async iterators are not available in node.js below v10, but there are some pretty good ways to consume a stream asynchronously without iterators, for example with scramjet:

Following the original examples, we can modify them for use with pg-promise:

Here's a sample which uses for-await-of to print "1", wait a second and then print "2":