isKeyof

The isKeyof() method checks whether a key exists in an object and narrows the key type accordingly.

Interactive example

Syntax

Classic signature

function isKeyof<
	GenericObject extends object,
	GenericKey extends ObjectKey
>(
	key: GenericKey,
	obj: GenericObject
): key is keyof GenericObject & GenericKey

Curried signature

function isKeyof<
	GenericObject extends object,
	GenericKey extends ObjectKey
>(
	obj: GenericObject
): (key: GenericKey) => key is keyof GenericObject & GenericKey

Parameters

• key: The key to check (string, number, or symbol).
• obj: The object in which to check for the key's existence.

Return value

A boolean indicating whether the key exists in the object. The return type uses a conditional type assertion to narrow the type of key when the condition is true.

See also

• includes - Checks whether a substring is present
• startsWith - Checks whether a string starts with a substring
• endsWith - Checks whether a string ends with a substring

Sources

• TypeScript - Type Guards - Documentation on type guards