WTF Typescript?! (The Standup)

I can't believe I'm watching a show called the standup and everybody is sat down. My disappointment is immeasurable.

I would really appreciate some sort of indicator in the video titles of Standup episode. Much love, great podcast.

I must admit that Casey is a win for this kind of videos. He is so invested into everything which makes the talks deep and interesting. Instead of just bashing on memes, he is trying to understand them and then decide. Super valuable to be part of this process. Thanks!

`(number | string)[]` is a heterogeneous array where any element can be number or string. `number[] | string[]` is a homogenous array where elements are either all numbers or all strings.

01:09:30 This is a well known topic in type systems called "type covariance". A container type is covariant if it preserves the subtyping relationships of contained types, i.e. since `number` is a subtype of `number | string` then `number[]` is a subtype of `(number | string)[]`. The issue is having covariant mutable container types is not sound (that's why everyone calls this a bug in TS). Languages that track mutability often have covariant immutable container types (no bug if you can't push to the array) but invariant mutable container types. Other languages have to choose either (a) don't have covariant container types, (b) have them but throw a runtime error when they're used in an unsound way, or (c) have them and just ignore the issue (like TS). It seems like since the TS type system is already unsound in many other ways (`any`, casting, etc) the TS team purposefully chose to have container types be covariant because it is useful in many cases and it's safe if you don't mutate the container.

I really want to witness Casey and the gang fix a TS compiler bug. Even if there's no pull request, just seeing a quick and dirty fix to a bug we're now all familiar with in compiler code would be sick.

Anyone wondering about that enum difference from 16:22: TLDR: There’s no reverse mapping (e.g., "valueA": "A") because string values are not guaranteed to be unique, and creating reverse mappings for strings could lead to ambiguity or complexity. For numeric enums, TypeScript creates a bidirectional mapping (a reverse mapping) between the enum member names and their values. This means you can access an enum member by its numeric value because TypeScript generates an object with both name-to-value and value-to-name mappings. enum NumericEnum { A = 1, B = 2, } TypeScript compiles this enum into an object like this: { A: 1, B: 2, 1: "A", 2: "B", } For string enums, TypeScript does not create a reverse mapping. String enums are compiled into an object that maps enum member names to their string values, but not the other way around. enum StringEnum { A = "valueA", B = "valueB", } TypeScript compiles the enum into an object like this: { A: "valueA", B: "valueB", }

Casey discovering how "const" works is hilarious

Casey’s lighting makes him look like Two-Face from the Batman animated series

Missed opportunity to call this “Everything AI Doesn’t Know About Typescript”