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

typeWorld="world";typeGreeting= `hello ${World}`;//typeColor="red"|"blue";typeQuantity="one"|"two";typeSeussFish= `${Quantity|Color} fish`;//typeVerticalAlignment="top"|"middle"|"bottom";typeHorizontalAlignment="left"|"center"|"right";// Takes//   | "top-left"    | "top-center"    | "top-right"//   | "middle-left" | "middle-center" | "middle-right"//   | "bottom-left" | "bottom-center" | "bottom-right"declarefunctionsetAlignment(value: `${VerticalAlignment}-${HorizontalAlignment}`):void;typeEnthusiasticGreeting<Textendsstring>= `${Uppercase<T>} - ${Lowercase<T>} - ${Capitalize<T>} - ${Uncapitalize<T>}`;typeHELLO=EnthusiasticGreeting<"heLLo">;// same as//   type HELLO = "HELLO - hello - Hello - heLLo";

This will require new AST nodes.

Key Remapping in Mapped Types

typeMappedTypeWithNewKeys<T>={[KinkeyofTasNewKeyType]:T[K]//            ^^^^^^^^^^^^^//            This is the new syntax!}

This will require AST changes.

Recursive Conditional Types

typeAwaited<T>=TextendsPromiseLike<inferU> ?Awaited<U> :T;

This will require no changes.

Pedantic Index Signature Checks

Example with thenoUncheckedIndexedAccess compiler option turned on:

functionscreamLines(strs:string[]){// this will have issuesfor(leti=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 likeno-unnecessary-condition for free!)

React 17 JSX Factories

This will require a slight tweak to how theparser passes options toscope-manager

Other changes that have no impact on us:

• paths withoutbaseUrl

• checkJs ImpliesallowJs

• Editor Support for the JSDoc@see Tag

• abstract Members Can’t Be Markedasync

  • we don't throw parser errors for invalid syntax (yetParsing: 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 inPromises