Function isFrozen

isFrozen<T extends object>(o: unknown): o is Readonly<T>
Determines whether the provided value is a frozen object.

Important Note:
JavaScript considers an object "frozen" if it is non-extensible and all of its properties are non-configurable and non-writable.

This leads to a subtle edge case:
- An empty object ({}) that is sealed or made non-extensible is also considered frozen, even if Object.freeze was never called.
Type Parameters
- T extends object
  The type of the input value.
Parameters
- o: unknown
  The value to be checked.
Returns o is Readonly<T>
true if the value is a frozen object, otherwise false.
Remarks
A frozen object is an object whose properties:
- cannot be added or removed
- cannot be reconfigured
- cannot be reassigned (all properties are effectively read-only)
This function only returns true for actual object values. It avoids the coercion behavior of Object.isFrozen, which returns true for all non-object (primitive) values.

Value Result Object.isFrozen

Object.freeze( {} ) true true

Object.seal( {} ) true true

{} false false

[] false false

null false true

undefined false true

123 false true
Since
1.0
- Defined in builtins/object.ts:369