Description
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:
-
pathswithoutbaseUrl -
checkJsImpliesallowJs -
Editor Support for the JSDoc
@seeTag -
abstractMembers Can’t Be Markedasync- 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/unknownAre Propagated in Falsy Positions -
--declarationand--outFileRequires a Package Name Root -
resolve's Parameters Are No Longer Optional inPromises