Support Typescript 4

Current Behavior

Creating a new project with the example code from the official release notes doesn’t work:

type Arr = readonly any[];

export function concat<T extends Arr, U extends Arr>(arr1: T, arr2: U): [...T, ...U] {
  return [...arr1, ...arr2];
}

$ yarn build
semantic error TS1256: A rest element must be last in a tuple type.

$ yarn lint
Parsing error: Cannot read property 'map' of undefined

Workaround

For the moment I managed to work around it by updating typescript and typescript-eslint and forcing the resolutions:

$ yarn add --dev typescript@latest @typescript-eslint/eslint-plugin@latest @typescript-eslint/parser@latest

// package.json

"resolutions": {
  "**/typescript": "^4.0.5",
  "**/@typescript-eslint/eslint-plugin": "^4.6.1",
  "**/@typescript-eslint/parser": "^4.6.1"
}

[utkarshkukreti]

For anyone interested, here’s the updated resolutions workaround I’m using for latest typescript and jest.

I’m using it in a new project and everything seems to be working fine; I’ll update if anything breaks.

  "resolutions": {
    "**/@typescript-eslint/eslint-plugin": "^4.11.1",
    "**/@typescript-eslint/parser": "^4.11.1",
    "**/jest": "^26.6.3",
    "**/ts-jest": "^26.4.4",
    "**/typescript": "^4.1.3"
  }

[stephenh]

We’re prototyping a new project on tsdx and after an ~hour of fighting “why does jsxImportSource” not work (for emotion setup), it turns out tsdx was using its own shadowed version of TypeScript. Pretty frustrating. Personally I’m close to moving off tsdx, but for now we can switch from npm to yarn and use the ^ resolutions.

All of the projects in our company are on TS 4.x and prettier 2.x, so kinda surprised to see tsdx behind on these.

Granted, it’s open source / it’s free / etc. 😃, so I’m looking forward to liking/using tsdx going forward, but just reporting back today’s frustrations from the field.