Array#isArray is too eager to infer unknown

[danielnixon]

What is the problem this feature would solve?

// Given something like this:
declare const maybeArray: number | readonly number[];

// Array.isArray is more difficult to use than it could be because it
// is too eager to infer its argument (and therefore return) type to unknown[]
// Type 'unknown' is not assignable to type 'number'.
// @ts-expect-error
export const definitelyArray: readonly number[] = Array.isArray(maybeArray) ? maybeArray : [maybeArray];

// Help inference with an explicit type param
export const definitelyArray2: readonly number[] = Array.isArray<typeof maybeArray>(maybeArray) ? maybeArray : [maybeArray];

What is the feature you are proposing to solve the problem?

Redefine isArray more like this:

declare const maybeArray: number | readonly number[];
declare const maybeArray2: unknown;

// If isArray was instead defined simply like this:
export const isArray = <T>(self: T): self is Extract<T, ReadonlyArray<unknown>> => Array.isArray(self)

// Then type inference would work without an explicit type argument
// const definitelyArray: readonly number[]
export const definitelyArray = isArray(maybeArray) ? maybeArray : [maybeArray];

// And the unknown case would still work as expected
// const definitelyArray2: unknown[]
export const definitelyArray2 = isArray(maybeArray2) ? maybeArray2 : [maybeArray2];

What alternatives have you considered?

No response

[danielnixon]

Oh, this is probably a dupe of #4051