TypeScript 4.0 Syntax Support

[bradzacher]

https://devblogs.microsoft.com/typescript/announcing-typescript-4-0-beta/

Variadic Tuple Types (Supported)

type Strings = [string, string];
type Numbers = number[]

// [string, string, ...Array<number | boolean>]
type Unbounded = [...Strings, ...Numbers, boolean];

This is already supported at the AST level - this is a type functionality change only.

Labelled Tuple Elements (Supported #2378)

type Foo = [first: number, second?: string, ...rest: any[]];

This requires AST updates.

Short-Circuiting Assignment Operators (Supported #2307)

// "Or Or Equals" (or, the Mallet operator :wink:)
a ||= b;

// "And And Equals"
a &&= b;

// "QQ Equals"
a ??= b;

This requires AST updates.
AST: estree/estree#212

unknown on catch Clause Bindings (Supported #2306)

try {
    // ...
}
catch (e: unknown) {
    // ...
}

This requires AST updates

[phiresky]

Support for the catch (e: unknown) syntax is in #2202 (together with a lint to enforce it)

[felixfbecker]

unknown on catch Clause Bindings

This would be awesome to have an ESLint rule for to require it! The TS team even alluded to that in their announcement :)

[felixfbecker]

With that it probably also makes sense to add a rule to enforce the same for Promise .catch() handlers

[bradennapier]

If v4.0 is live - should the docs be updated? They still say that it only supports TypeScript < 3.10.0

[tilgovi]

@bradennapier still only v4.0.0-rc, not stable release yet.

[bradennapier]

@bradennapier still only v4.0.0-rc, not stable release yet.

Sweet, thanks! Well the docs on that branch I think still say the support is for < 3.10.0 if i am not mistaken ;-) - just one easy to overlook thing to double check!

Thanks!

[bradzacher]

Whilst we support the syntax, we don't officially mark ourselves as supporting the beta/rc versions.
https://github.com/typescript-eslint/typescript-eslint#supported-typescript-version

This release required minimal changes to support, so we were luckily able to add full support ahead of time. 3.9 was similar in that it required no changes, but we waited for the full release before bumping. 3.8 we still don't have full support for because we're waiting on the AST spec for private class members.

Note that our v4 release is entirely unrelated to TS v4 release (I wasn't supposed to link that PR, I wasn't thinking and confused myself).

When TS v4.0 officially releases, there may be some changes since the RC that we need to handle, so we have to wait until it is released before we update all of the docs and version ranges.

[pattrickrice]

checking in here since I'm doing an upgrade right now and see the warnings from this package in my console
https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/

[bradzacher]

Well yeah, TS4.0 was only released to npm 3 hours ago.......
We'll bump the ranges and it'll be released as part of our standard release cycle.

[felixfbecker]

Can't wait 😍

[Retsam]

It looks like TS 4.0 is supported, according to the status of this ticket, but I've upgraded TS to 4.02, and @typescript-eslint/eslint-plugin and @typescript-eslint/parser to 3.10.1, but I'm getting Parsing error: Cannot read property 'map' of undefined on any file that uses tuple type syntax, (with or without labels).

Basically I'm seeing #2098 (but that's been closed and locked, otherwise I'd comment over there).

Am I missing something obvious here?

[bradzacher]

likely your versions are out of sync somewhere - either there are multiple versions of our tooling installed locally, or your IDE needs to be restarted to pick up the new versions.

We have passing tests (fixture and snapshot) that are currently running against TS4.0.2, so it's unlikely to be a bug from our end? Unless I missed some syntax.

[neilbryson]

This happened to me as well, first in CI builds only but eventually in my local machine after deleting node_modules and reinstalling packages. Not sure what's wrong.

[bradzacher]

I'm unable to repro. As mentioned it's working fine in master.

Also I just tried a clean install with:

    "@typescript-eslint/parser": "^3.10.1",
    "typescript": "^4.0.2"

// .eslintrc.js
module.exports = {
  parser: "@typescript-eslint/parser",
  rules: {
    semi: 1
  }
};

// a.ts
type T = [a: string];

$ DEBUG=typescript-eslint:* yarn eslint a.ts
  typescript-eslint:typescript-estree:createIsolatedProgram Getting isolated program in TSX mode for: ./a.ts +0ms
Done in 0.96s.

Check the versions installed in your yarn.lock. Likely there are multiple versions installed in your workspace.

[neilbryson]

Oh yeah, you're right. react-scripts@3.4.3 still depends on ^2.10.0. Tried forcing a yarn resolution on it, and it compiled successfully.

Thanks!

[JohannesKlauss]

@neilbryson How did you force that resolution on react-scripts?

[bradzacher]

Yarn has this functionality built in

https://classic.yarnpkg.com/en/docs/selective-version-resolutions/