Array.isArray no longer narrows 'object' values in 5.1

[AustinGrey]

Bug Report

🔎 Search Terms

All open issues referencing "Array.isArray"
"typeguarding arrays from object values"

🕗 Version & Regression Information

• This changed between versions 5 and 5.1

⏯ Playground Link

Playground link with relevant code

💻 Code

export function searchObject<T extends object>(
	obj: T,
	properties: (keyof T)[],
) {
	for (const property of properties) {
		const val = obj[property];
		if (Array.isArray(val)) {
			val.forEach(() => {});
		}
	}
}

🙁 Actual behavior

val may be an array, but Typescript reports that forEach is not defined on val because T[keyof T] cannot be an array.

🙂 Expected behavior

val is 'narrowed' to T[keyof T] & any[]

[typescript-bot]

The change between origin/release-5.0 and origin/release-5.1 occurred at 84a09c7.

[RyanCavanaugh]

The originating change is being reverted. Track at #54845

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.