isValueOf

Checks whether a value exists among the values of a given object. This helper works with any plain object, including objects with a null prototype.

It never throws an error and never mutates the input. Use it when you need to confirm that a value is one of the values of a particular object.

Signature

function isValueOf<T extends Record<PropertyKey, unknown>>(
  obj: unknown,
  value: unknown
): obj is T

Parameters

Name	Data type	Description
object	T	The object to check keys against. Must be a non-null object.
key	unknown	The value to check as a potential key.

Returns

Boolean:

true: If value exists among the values of obj, false otherwise. false: If obj is null or a primitive.

Works for any plain object, including objects created with Object.create(null).

Behavior

• The function performs an exact equality check (===) against each value in the object.
• Never throws and never mutates input.
• Suitable for strict type narrowing: after if (isValueOf(obj, value)) { … }, TypeScript knows obj is of type T.

Examples

const obj = { a: 1, b: 2, c: "hello" };

isValueOf(obj, 1); // true
isValueOf(obj, "hello"); // true

const nullProto = Object.create(null);
nullProto.x = 123;

isValueOf(nullProto, 123); // true


isValueOf(obj, 3); // false
isValueOf(null, 1); // false
isValueOf(42, 1); // false

Notes

• Does not coerce types. "1" will not match 1, and boxed primitives (e.g., Object(1)) are treated as objects, not numbers.
• Works correctly with empty objects ({}) and null-prototype objects.
• Designed for pure, functional usage: it never mutates the input object and never throws exceptions.