TypeScript 4.0 Iteration Plan

[DanielRosenwasser]

This document outlines our focused tasks for TypeScript 4.0, as well as some of the discussion that explains how/why we prioritized certain work items. Nothing is set in stone, but we will strive to complete them in a reasonable timeframe.

Date	Event
May 12th	TypeScript 3.9 Release (past)
June 22nd	Create 4.0 Beta (4.0.0) Build for Testing
June 25th	TypeScript 4.0 Beta Release
July 31st	Create 4.0 RC (4.0.1) Build for Testing
August 6th	TypeScript 4.0 RC Release
August 14th	Create 4.0 Final (4.0.2) Build for Testing
August 20th	TypeScript 4.0 Final Release 🚀

Language Features

• Short-Circuiting Assignment Operators
• Labeled Tuple Elements
• Class Property Inference from Constructors
• Forward Declarations
• Investigate the awaited Type
• JSX Factory Updates
• unknown on catch Clause Bindings
• Variadic Tuples

Editor Productivity

• Refactorings
  • Convert to Optional Chaining and Coalescing
• Discoverability in Refactoring (ideas)
• Lightweight Navigation
• Improved Auto-Import
• Editor support for /** @deprecated */ tags
• Editor support for /** @see */ tags
• Semantic Highlighting

Performance

• More Type-Checking Optimizations
• Investigate Bottlenecks in Larger Apps

Infrastructure

• Migrate TypeScript to Modules
• Expand Commands Tested in TSServer Crawler
• GitHub Crawler Infrastructure to Find Breaking Changes

Investigate High-Demand Bug Fixes

• Don't Widen Computed Property Keys
• Cleaning Files Under --watch
• Higher Order Component Issues
• Remove --declaration Emit Error
• Inference for Proxies
• change events on <input type="file">have no files field
• Poor Error for isolatedModules

[checkmatez]

Would you consider to include #29818 in 4.0 maybe behind experimental flag?

[Jessidhia]

That gets complicated by the change in factory function definition in itself.

It could be interesting to introduce a separate factory virtual type definition, though; say, JSX.Factory, or React.JSX.Factory, which TypeScript could then use for inference. I'm not quite sure just translating JSX grammar to function calls is sufficient or efficient but, since it's a virtual type, it doesn't have to correspond to any concrete JavaScript entity. The risk, of course, is getting into the same situation as we have now, with a bunch of virtual types that ended up limiting the type safety, not only of children, but of several features introduced after React 15.

[ExE-Boss]

Would you consider to also include #24738 in 4.0 as well?

[leemhenson]

I'm a bit sad to see no mention of #33038 [👍 140 and an actual PR by your own @weswigham] or #202 [👍 390] while tickets like #15230 [👍 27] are considered "high-demand". I realise you can't really compare or prioritise on the basis of "likes" but it would be great if there was some roadmap update on these, especially as 4.0 seems like nice opportunity to introduce a feature like this. 🙏

[Skillz4Killz]

~3.5-year-old issue awaiting feedback with almost 200 comments #13778 with inaccurate typings provided for stuff like array destructuring. Pwetty pwease can we implement fix 🙏

[DanielRosenwasser]

I appreciate people occasionally boosting issues that they believe need attention on the roadmaps and iteration plans, but I think I need to be clear here -that "investigate high demand bug fixes" section was determined by looking through issues that were clearly causing a lot of papercuts but which seemed reasonable in scope. We're definitely still mindful of the issues mentioned, but some of them are not as scoped nor do they have a clear ideal outcome.

Examples:

• Nominal brands would be nice, but would that compose with future language direction around nominality? I even have this concern with placeholder types as the person who proposed it.
• undefined on index signatures is an example of something that's interesting, but we don't want to add behavior that makes it harder for 90% of people who already operate under the current assumptions. Finding an approach that composes and allows users to incrementally adopt those checks isn't something that's obvious to us. Even if it's technically possible with a bunch of conditional types and special compiler checks, those solutions tend to be very clearly hacky and break down fast.

[aminpaks]

Nominal brands would be nice, but would that compose with future language direction around nominality?

@DanielRosenwasser what is the future language direction in your vision?

[DanielRosenwasser]

I guess I'll give some context on where my mind is with nominality. There are a lot of different ideas people have in mind when they ask about nominal types, including

• "Traditional" declaration-based nominality (e.g. what you see in most OO languages)
• Opaque types (types whose contents are entirely unknown outside)
• Distinct aliases (single-member structs in C/C++/C#, newtype in Haskell, inline classes in Kotlin)
• Units of measure (a way to encode dimensional analysis into the language)

There are shades between some of these (e.g. placeholder type declarations - kind of variant of opaque types that fall back to an implementation type), and then there are different directions that blend each of these together.

@RyanCavanaugh had a great analogy about this where 3 kids are asking their parents for a pet. One wants a dog, one wants a cat, one wants a fish. They ask their parents "when are we getting a pet!?" Clearly they all agree they want a pet, but each wants a different pet!

Do I like branded types? I do! Branded types achieves something like distinct aliases, and fits the bill for what most users are looking for. But I don't think that's the right way to think about it. There's more design space to be fleshed out with plenty of known tradeoffs, and nothing giving me a sense that we need to rush a solution ASAP.

[ExE-Boss]

I’d like if we could get microsoft/TypeScript-DOM-lib-generator#858 into TypeScript 4.0.

[benjamingr]

@DanielRosenwasser first of all thank you for the writeup 🙇

Can you elaborate a bit on what use cases nominal types actually address for TypeScript?

First: feel free to send me to a giant wall of text or a repo and I'll read it :]

I don't mean opaque types like placeholder types - I mean what you call "Traditional" nominal types.

I always felt nominal types were antithetical to JavaScript and that's why previous attempts didn't really work so well. There are ways to make it work pretty nicely (protocols in swift and typeclasses in haskell come to mind as "Nominal but extendable from the outside") and I'm sure you're familiar with most of the "well established" ways (I assume "units of measure" is an F# wink).

I have found a lot of people asking for nominal (as in "traditional") types but not a lot of writeups about why.

[DanielRosenwasser]

Over time, we've seen fewer and fewer people request "traditional" nominal types, maybe because collectively the community has built up a mental mode for structural types. There are some places where types truly do act nominally (when instanceof is involved or when there are privates). Some of that is captured with better control flow analysis and compatibility checks, but it's not perfect.

Some of the "traditional" use-cases are the same as those of a zero-overhead nominal wrapper type (e.g. newtype), and a lot of the time the intent there is to ensure special handling for things like file paths, untrusted strings, etc.

[chyzwar]

I kind of feel that Typescript is already expressive enough. What I would personally would like to see is better tooling integration.

• support for ESM New --emitExtension and --noImplicitExtensionName compiler options #35148 Provide a way to add the '.js' file extension to the end of module specifiers #16577
• compiler plugins Allow "Compiler Plugins" #16607 Pre-Compiler Plugin Proposal #38736
• yarn pnp Add new moduleResolution option: yarn-pnp #28289 Native support for PnP #35206

Because of above it is common to see clunky and hacky solutions. Most react projects have babel included for react-hot-loader (compiler plugins), some CSS systems also require babel for compile time transforms. Using pnp, esm or even CSS modules require more tooling and workarounds for tsc limitations.

It is also frustrating that for some of these issues community came with concrete solutions in form of PRs or proposals but these were rejected or stalled for years. As a practitioner, it is getting harder to use TS in the context of wider ecosystem.

Anyway I am just random person from internet.

[JoshuaKGoldberg]

@DanielRosenwasser maybe #29374 could get reviewed in time for 4.0? I think it covers many (most?) of the this-before-super cases folks tend to ask about.

[mathiasrw]

Please reconsider including support for referencing ES modules with a file path including the file extension. It will give a big boost to level the playing field of multi-environment code.

#16577

[qlonik]

Thank you for your focus on the tooling around typescript! I was hoping you could consider providing closer integration with https://github.com/microsoft/tsdoc. A lot of other modern languages like go and rust provide documentation tooling right out of the box and encourage developers to write standard comments, but typescript is lacking in this regard.

[zen0wu]

It would be super great if #31445 can be picked up, this has been a deal breaker for us and I believe a lot of people (based on how many references that issue has)!

[ExE-Boss]

Can #38967 be included in TypeScript 4.0, and maybe also #35608.

[Jack-Works]

I found people are requesting everything to be included in the 4.0 release 😆

[canonic-epicure]

Feels like TS is loosing momentum? Nullish coalescing and short-circuiting assignments for the next, major, 4.0.0 release? No big goals and ambition ideas anymore?

For the next major, I'd expect:

• Higher kinded typing
• Dependent typing? Type-level functions? (Ok, this one can be for 5.0.0)
• Nominal typing
• Macroses
• Support for transformers in tsconfig.json
• Compilation to WASM (of some language subset)

[Shinigami92]

@canonic-epicure Agree, currently it feels more like a 3.10 for the next version

[Jack-Works]

Feels like TS is loosing momentum? Nullish coalescing and short-circuiting assignments for the next, major, 4.0.0 release? No big goals and ambition ideas anymore?

TypeScript doesn't follow the semver. There isn't v0.10, v1.10, v2.10, or v3.10. So this is actually a normal milestone. It's a bit strange people are expecting so many changes in 4.0 but not saying anything in 3.9 or prior iteration plan. 🙈

[Jack-Works]

Type-level functions

type X<T> = T can do it on some level. (not supporting higher-order functions.)

Macroses

IMO it does not satisfy the Typescript's goal.

Support for transformers in tsconfig.json

Have a try: https://github.com/cevek/ttypescript

Compilation to WASM (of some language subset)

Do you looking for https://www.assemblyscript.org/

[canonic-epicure]

Ok, so 4.0.0 is just a next minor release, good to know.

Regarding the "macroses do not satisfy the TS goal" and "use ttypescript for transformers" (ts-patch works better btw) - this resembles the jedi move - "this is not the feature you need". No, macroses and transformers out of the box please. Its been requested years ago.

[Jack-Works]

I also want marco in TypeScript but there're really many reasons against this.

1. if Marco can generate different JavaScript code, it's creating a new non-type level syntax, therefore it's an extension to the ES spec. enum, import x = require(...), module is the exception of this rule but they come from the early time of TypeScript.

2. if you want to use Marco cross different files to generate different JS files, then it's impossible to dumbly drop all type syntax to get the JavaScript file.

3. Marcos can increase the analysis & compile time, and likely to be abused.

4. If Marco can emit different JS files based on the type info, it will make Babel impossible to handle this syntax. So this behavior is violating the isolatedModules rule (exception: const enum and module)

5. If Marco can only do a "type level Marco" and can be erased by Babel safely, it becomes much useless but still useful for higher-order / programmatic types. Therefore it is not violating any goals, but I'm doubting if the TypeScript team will interest in the idea. (I have made a demo at https://github.com/Jack-Works/typescript-marco-demo/blob/master/marco-test.ts).

[canonic-epicure]

@Jack-Works I don't understand your points. Perhaps you mean that macroses will extend parser and create new synatx?

For me, macros is just AST -> AST function, that runs prior typechecker somehow, therefor it can create new AST nodes that will be typechecked regularly. It does not create new syntax however - the input is regular AST, the output too. It does create new nodes in AST.

The discussion will be off-topic for this thread, perhaps to be continued on #compiler channel in discord?

[xiaoxiangmoe]

Can #38597 be included in TypeScript 4.0 ？

[DanielRosenwasser]

@weswighammmmmmmmmmmmmmmmmmm, the bot hates me ): ): ): ):

Absolutely no rush, but I did it manually and filed this: #39869

[timreichen]

Is there already a roadmap for after 4.0 somewhere? I would like to boost #37582 since there is a more and more growing demand (#39965, #38149, #27481, #39965, #38546). It looks like a reasonably scoped issue to me.

[DanielRosenwasser]

Just as an update, we had to move over the RC by 2 days for the website launch and for other changes we felt we needed to land in the RC. We don't anticipate anything pushing us back much further, but to be safe, we'll be giving ourselves another 2 days to get feedback on the RC, and aiming for the 20th instead.

[DanielRosenwasser]

@typescript-bot bump release-4.0

[typescript-bot]

Heya @DanielRosenwasser, I've started to update the version number on release-4.0 to 4.0.2 for you. Here's the link to my best guess at the log.

[Aravin]

it's Aug 20 :)

[DanielRosenwasser]

Heh, it's still August 19th here, and even 20 minutes from now I think you'll have to wait a bit more. We have nightly releases on npm if you can't take another minute! 😄

[Ayfri]

Waiting for the release, already out in npm :)

[DanielRosenwasser]

@typescript-bot bump release-4.0

[typescript-bot]

Heya @DanielRosenwasser, I've started to update the version number on release-4.0 to 4.0.3 for you. Here's the link to my best guess at the log.

[DanielRosenwasser]

@typescript-bot bump release-4.1

[typescript-bot]

Heya @DanielRosenwasser, I've started to update the version number on release-4.1 to 4.1.1-rc for you. Here's the link to my best guess at the log.

[DanielRosenwasser]

Oh no

[DanielRosenwasser]

@typescript-bot bump release-4.0

[typescript-bot]

Heya @DanielRosenwasser, I've started to update the version number on release-4.0 to 4.0.4 for you. Here's the link to my best guess at the log.

[DanielRosenwasser]

@typescript-bot bump release-4.0

[typescript-bot]

Heya @DanielRosenwasser, I've started to update the version number on release-4.0 to 4.0.5 for you. Here's the link to my best guess at the log.

[DanielRosenwasser]

@typescript-bot bump release-4.0

[typescript-bot]

Heya @DanielRosenwasser, I've started to update the version number on release-4.0 to 4.0.8 for you. Here's the link to my best guess at the log.