Inferred Type Predicates - TypeScript 5.5's top new feature

This has has been a top wishlist features for so many people for so many years. Dan tells the story of how he cracked it, then we do a deep dive on the code itself. Although this was a pretty technical conversation there ended up also being quite a lot of great conversation on staying motivated and thinking through all the aspects of the problem from side-effects to performance implications.

Links:

Dan Vanderkam is an independent software developer, formerly at Google and Sidewalk Labs. He is the author of numerous open source JavaScript/TypeScript libraries, including the dygraphs charting library, source-map-explorer, literate-ts, pg-to-ts and crosswalk. He is the author of "Effective TypeScript: 83 Specific Ways to Improve Your TypeScript" (O'Reilly 2024) and has spoken at numerous TS conferences. When he's not writing about TypeScript, Dan enjoys hiking, rock climbing and birding.

00:00:00 in this episode
00:02:06 the Recurse Center (deciding to tackle this issue)
00:03:53 first issue: numeric literal separators in ES2021
00:07:36 rediscovering this old issue
00:08:28 explaining the fundamentals of the feature
00:13:43 the idea for the first approach to solving the issue
00:16:24 wrong assumptions about how the compiler handles inference
00:17:59 what is a "Flow Node" in the compiler
00:21:46 about the TypeScript AST Viewer
00:22:18 second attempt: using Flow Nodes
00:23:26 digging into the code for the actual implementation
00:28:03 what most people don't understand about how type predicates actually work
00:30:21 are MiTS viewers interested in blog post readings (WITH the author)?
00:31:33 looking deeper than a type predicate goes
00:33:23 working with TypeScript's exhaustive test suite
00:36:03 dealing with outlier falsey types (like empty string, NaN, etc.)
00:38:27 what about `[].filter(Boolean)` (array filter with the Boolean constructor)
00:39:14 type guards now flow between functions
00:40:35 sage-tier advice from Anders Hejlsberg
00:41:20 building the courage to submit the PR
00:43:59 a scary moment where it looked like the solution might not actually work
00:46:37 a solution to the setback after going for a walk
00:50:29 a totally unique approach within the TypeScript compiler codebase
00:53:12 the process for getting it reviewed and approved and merged
00:54:36 performance optimization work
00:56:42 playing around with the end result
00:57:24 playing with truthiness checks
00:59:15 revisiting: why you shouldn't use .filter(Boolean)
01:01:57 about teaching these concepts
01:02:52 how ts-reset handles filter(Boolean)
01:05:51 wrapping up
01:07:49 after it all: it turns out this could have been done years ago

[MichiganTypeScript]

PR author interviews like this have been something we've been wanting to do for a long long time, and we're so excited to finally get it started! Let us know what you think of this format and what kinds of PRs you'd like to see in the future.

[hansschenker]

I like the format, but It is only as good as the people are. It must be people of the format of Dan or Josh! If you can catch them do it!

[JoeyGravlin]

Amazing interview, learned a lot. Dan has some great insights and MiTS coming in clutch with thoughtful questions. Stay gold.

[kasper_]

Great interview! It’s so fun to see the journey of how something grew into its final form.