Add additional overloads to Array.prototype.concat #26976
Description
Search Terms
concat never
Suggestion
Add the following signatures to the Array interface:
interface Array<T> {
concat<U>(this: Array<never>, ...items: ConcatArray<U>[]): U[];
concat<U>(this: Array<never>, ...items: (U | ConcatArray<U>)[]): U[];
}For my use case this is not needed in ReadonlyArray, but could be added for consistency.
Use Cases
I often find myself in the need of using Array.prototype.flatMap, but since I target Node.js 6 it's not available. Therefore I try to replace it with [].concat(someArray.map(mightReturnArray)).
But then I get an error like Type 'whatever' is not assignable to type 'never'. (with strictNullChecks enabled).
Examples
interface Array<T> {
/**
* Combines two or more arrays.
* @param items Additional items to add to the end of array1.
*/
concat<U>(this: Array<never>, ...items: ConcatArray<U>[]): U[];
/**
* Combines two or more arrays.
* @param items Additional items to add to the end of array1.
*/
concat<U>(this: Array<never>, ...items: (U | ConcatArray<U>)[]): U[];
}
// now OK
[].concat(['a']);
['a'].concat(['b']);
// still an error as expected
[1].concat(['a']);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. new expression-level syntax)