Should this be an RFC?

• This is not a substantial change

Which package is this a feature request for?

Task (@lit/task)

Description

TypeScript 5.0 added the const modifier for type variables.

It can be adapted in@lit/task like so:

export class Task<
-  T extends ReadonlyArray<unknown> = ReadonlyArray<unknown>,
+  const T extends ReadonlyArray<unknown> = ReadonlyArray<unknown>,
-  R = unknown,
+  const R = unknown,
 > {

The advantage of this is that argument types and return types would be inferred better by TypeScript. Example:

// Before this was inferred as Task<(string | undefined)[], string[]>
// With addition of "const" it would be Task<readonly [string | undefined], readonly [string]> - a much more accurate type
  _dataTask = new Task(
    this,
    async ([name]) => {
      if (name === undefined) {
        return "";
      }

      const response = await fetch(`example.com/${name}`);
      return [await response.text()];
    },
    () => [this.name],
  );

Alternatives and Workarounds

If const is not included in the @lit/tasks typings, it can be manually included in the component source code:

-    () => [this.name],
+    () => [this.name] as const,

however, not every TypeScript user would be aware of this, so some might get frustrated by bad type checking experience when using Lit Task