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Generics interface conditional type #40104
Description
Search Terms
Generics conditional type
Suggestion
interface Array<T> {
// allow for all type
some(callbackfn: (value: T, index: number, array: T[]) => unknown, thisArg?: any): boolean;
}
interface Array<T when T is number> {
// only when `T` is number
sum(): number;
avg(): number;
max(): number;
min(): number;
}Use Cases
const a = ['a', 'c', 'd'];
// a.sum() // not allowed
const b = [1, 2, 5];
const c = b.sum(); // ok
console.log(c); // 8Examples
Checklist
My suggestion meets these guidelines:
- This wouldn't be a breaking change in existing TypeScript/JavaScript code
- This wouldn't change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
- This feature would agree with the rest of TypeScript's Design Goals.
Activity
[-]Generics conditional type[/-][+]Generics interface conditional type[/+]MartinJohns commentedon Aug 18, 2020
What about
Array<string | number>?mokeyish commentedon Aug 18, 2020
I write some extension methods for
Array, but it is not work as expectedly.mokeyish commentedon Aug 18, 2020
Currently , interface declarations with the same qualified name, it
Declaration Merginghttps://github.com/microsoft/TypeScript/blob/master/doc/spec.md#7.2
MartinJohns commentedon Aug 18, 2020
Please note that the spec is completely outdated and abandoned.
RyanCavanaugh commentedon Aug 19, 2020
You can already do this today; no new syntax is necessary:
mokeyish commentedon Aug 19, 2020
Ryan Cavanaugh (@RyanCavanaugh) Thanks!😁
