TypeScript 4.1 Syntax Support

[bradzacher]

https://devblogs.microsoft.com/typescript/announcing-typescript-4-1-beta/

This issue is just to track all of the new features and their implementation state in this project.
As with all releases, we will not necessarily to support all features until closer to the full release when everything the features are stabilised.
Please be patient.

---

Template Literal Types

type World = "world";
type Greeting = `hello ${World}`;

//

type Color = "red" | "blue";
type Quantity = "one" | "two";
type SeussFish = `${Quantity | Color} fish`;

//

type VerticalAlignment = "top" | "middle" | "bottom";
type HorizontalAlignment = "left" | "center" | "right";

// Takes
//   | "top-left"    | "top-center"    | "top-right"
//   | "middle-left" | "middle-center" | "middle-right"
//   | "bottom-left" | "bottom-center" | "bottom-right"
declare function setAlignment(value: `${VerticalAlignment}-${HorizontalAlignment}`): void;

type EnthusiasticGreeting<T extends string> = `${Uppercase<T>} - ${Lowercase<T>} - ${Capitalize<T>} - ${Uncapitalize<T>}`;
type HELLO = EnthusiasticGreeting<"heLLo">;
// same as
//   type HELLO = "HELLO - hello - Hello - heLLo";

This will require new AST nodes.

Key Remapping in Mapped Types

type MappedTypeWithNewKeys<T> = {
    [K in keyof T as NewKeyType]: T[K]
    //            ^^^^^^^^^^^^^
    //            This is the new syntax!
}

This will require AST changes.

Recursive Conditional Types

type Awaited<T> = T extends PromiseLike<infer U> ? Awaited<U> : T;

This will require no changes.

Pedantic Index Signature Checks

Example with the noUncheckedIndexedAccess compiler option turned on:

function screamLines(strs: string[]) {
    // this will have issues
    for (let i = 0; i < strs.length; i++) {
        console.log(strs[i].toUpperCase());
        //          ~~~~~~~
        // error! Object is possibly 'undefined'.
    }
}

This will require no changes (it will just start working and improve existing rules like no-unnecessary-condition for free!)

React 17 JSX Factories

This will require a slight tweak to how the parser passes options to scope-manager

---

Other changes that have no impact on us:

• paths without baseUrl

• checkJs Implies allowJs

• Editor Support for the JSDoc @see Tag

• abstract Members Can’t Be Marked async

  • we don't throw parser errors for invalid syntax (yet Parsing: strictly enforce the produced AST matches the spec and enforce most "error recovery" parsing errors #1852)

• any/unknown Are Propagated in Falsy Positions

• --declaration and --outFile Requires a Package Name Root

• resolve's Parameters Are No Longer Optional in Promises

[aggmoulik]

Hy @bradzacher I am working on Specs for the template literal type. If there anything you want me to consider before creating PR.

[bradzacher]

Nothing really. Assuming it should be pretty simple to setup. I was kind of considering an AST like:

interface TSTemplateLiteralType extends Node {
  type: AST_NODE_TYPES.TemplateLiteralType;
  quasis: TemplateElement[];
  expressions: TypeNode[];
}

So that it mirrors the standard template literal.

[ranneyd]

Status on this?

[bradzacher]

https://github.com/typescript-eslint/typescript-eslint#supported-typescript-version
TS4.1 is still in beta, and as such is still not stable. We will not be rushing to explicitly support it for the moment.

[mauriziocarboni2]

From the Iteration plan we can see that TS4.1 will have a Release Candidate on November 3rd.

microsoft/TypeScript#40124

can we expect this issue to be fixed shortly after?

[bradzacher]

TS4.1 just hit RC!
https://devblogs.microsoft.com/typescript/announcing-typescript-4-1-rc/

I'll look to filling in support soon.

[dipasqualew]

With TypeScript 4.1 released, this issue is still closed, but I believe ESlint still breaks with:

from my package.json:

    "@typescript-eslint/eslint-plugin": "^4.8.2",
    "@typescript-eslint/parser": "^4.8.2",
    "eslint": "^7.14.0",
    "typescript": "^4.1.0"

[mlippert]

With TypeScript 4.1 released, this issue is still closed, but I believe ESlint still breaks with:

@dipasqualew,
Hmm, other than "typescript": "^4.1.2" I've got the same dependencies but it seems to work ok for me, so what is breaking for you?

Also I think the Supported Typescript Version on the readme can now be updated to state <4.2.0 instead of <4.1.0 since this tracking issue is closed, right?

[bradzacher]

The readme is already up to date!

There are no breakages for TS4.1 - we have many passing tests.

If you think you have an issue, please file a new issue and fill out the template - as per our contributing guide.

https://github.com/typescript-eslint/typescript-eslint/blob/master/CONTRIBUTING.md

Help me to help you.