Not possible to narrow records down when wanting to listen to everything

[Jiralite]

Description

It does not seem possible to narrow down the union of records when wanting to listen to everything:

import { CommitType, EventType, Jetstream } from "@skyware/jetstream";

const jetstream = new Jetstream();

jetstream.on(EventType.Commit, (event) => {
	const commit = event.commit;

	if (commit.operation === CommitType.Create) {
		const record = commit.record;

		if (record.$type === "app.bsky.feed.post") {
			record.text; // Property 'text' does not exist...
		}
	}
});

jetstream.start();

I would have expected record to be of type AppBskyFeedPost.Record, but it was still AppBskyActorProfile.Record | AppBskyFeedGenerator.Record | AppBskyFeedLike.Record | AppBskyFeedPost.Record | AppBskyFeedPostgate.Record | AppBskyFeedRepost.Record | AppBskyFeedThreadgate.Record | AppBskyGraphBlock.Record | AppBskyGraphFollow.Record | AppBskyGraphList.Record | AppBskyGraphListblock.Record | AppBskyGraphListitem.Record | AppBskyGraphStarterpack.Record | AppBskyLabelerService.Record | ChatBskyActorDeclaration.Record.

tsconfig.json:

{
	"compilerOptions": {
		"strict": true,
		"module": "NodeNext",
		"moduleResolution": "NodeNext",
		"lib": ["ESNext"],
		"target": "ESNext",
	}
}

For now, just doing "text" in record suffices, I guess... unless I am being oblivious to something? If so, please let me know!

As a side note, I must skip the library check when type checking because it gives compile errors from partysocket. It's not strictly relevant to this package I guess.

index.ts:12:11 - error TS2339: Property 'text' does not exist on type 'AppBskyActorProfile.Record | AppBskyFeedGenerator.Record | AppBskyFeedLike.Record | AppBskyFeedPost.Record | AppBskyFeedPostgate.Record | AppBskyFeedRepost.Record | AppBskyFeedThreadgate.Record | AppBskyGraphBlock.Record | AppBskyGraphFollow.Record | AppBskyGraphList.Record | AppBskyGraphListblock.Record | ... 4 mor...'.
  Property 'text' does not exist on type 'Record'.

12    record.text;
             ~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:5:189 - error TS2304: Cannot find name 'AddEventListenerOptions'.

5     addEventListener<K extends keyof EventMap>(type: K, callback: (event: EventMap[K] extends Event ? EventMap[K] : never) => EventMap[K] extends Event ? void : never, options?: boolean | AddEventListenerOptions): void;
                                                                                                                                                                                              ~~~~~~~~~~~~~~~~~~~~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:6:46 - error TS2304: Cannot find name 'EventListenerOrEventListenerObject'.

6     addEventListener(type: string, callback: EventListenerOrEventListenerObject | null, options?: EventListenerOptions | boolean): void;
                                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:6:99 - error TS2304: Cannot find name 'EventListenerOptions'.

6     addEventListener(type: string, callback: EventListenerOrEventListenerObject | null, options?: EventListenerOptions | boolean): void;
                                                                                                    ~~~~~~~~~~~~~~~~~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:7:192 - error TS2304: Cannot find name 'AddEventListenerOptions'.

7     removeEventListener<K extends keyof EventMap>(type: K, callback: (event: EventMap[K] extends Event ? EventMap[K] : never) => EventMap[K] extends Event ? void : never, options?: boolean | AddEventListenerOptions): void;
                                                                                                                                                                                                 ~~~~~~~~~~~~~~~~~~~~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:8:49 - error TS2304: Cannot find name 'EventListenerOrEventListenerObject'.

8     removeEventListener(type: string, callback: EventListenerOrEventListenerObject | null, options?: EventListenerOptions | boolean): void;
                                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:8:102 - error TS2304: Cannot find name 'EventListenerOptions'.

8     removeEventListener(type: string, callback: EventListenerOrEventListenerObject | null, options?: EventListenerOptions | boolean): void;
                                                                                                       ~~~~~~~~~~~~~~~~~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:75:23 - error TS2304: Cannot find name 'BinaryType'.

75     get binaryType(): BinaryType;
                         ~~~~~~~~~~

node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:76:27 - error TS2304: Cannot find name 'BinaryType'.

76     set binaryType(value: BinaryType);
                             ~~~~~~~~~~


Found 9 errors in 2 files.

Errors  Files
     1  index.ts:12
     8  node_modules/.pnpm/[email protected]/node_modules/partysocket/ws.d.ts:5

Versions

• Node.js 22.12.0
• @skyware/jetstream 0.2.2
• @types/node 22.10.5
• typescript 5.7.3

[futurGH]

This is difficult unfortunately — it's possible to receive records with an unknown $type and basically any shape, so the types need to account for that. It's effectively a version of microsoft/TypeScript#26277.

You can somewhat work around it with a type guard like this:

type ExtractByType<Record extends { $type: string }, Type extends string> = 
    Record extends Record
        ? string extends Record["$type"] ? never : Record & { $type: Type }
        : never;

function is<Type extends string, T extends { $type: string }>(record: T, type: Type): record is ExtractByType<T, Type> {
    return record.$type === type;
}

if (is(record, "app.bsky.feed.post")) {
	record.text;
}

I'm not sure I'll add this into the library, so I'll leave this issue up for other users to find.

[estrattonbailey]

Another option would be to pull the the atproto lexicon utils:

import {AppBskyFeedPost} from '@atproto/lexicon'

if (AppBskyFeedPost.isRecord(record)) {
  record.text; // => type 'string'
}

But that only checks the $type value. If you want to actually validate the data (using Zod):

if (AppBskyFeedPost.validateRecord(record).success) {
  record.text; // => type 'string'
}

The latter is more of a perf hit though, so avoid it in critical paths. If you're reading data from our APIs or firehose, you can trust the more simple and fast isRecord checks.

[futurGH]

Only problem is that this library uses Mary's atcute, not @atproto/api :) Mainly because the latter didn't have a type-level mapping from lexicon name to record type until Matthieu added it fairly recently iirc?

[estrattonbailey]

Oh my mistake! Sorry about that 😅