Function isSealed

isSealed<T extends object>(o: T): o is T

Determines whether the provided value is a sealed object.

Important Note:
Empty objects ({}) that are made non-extensible (via Object.preventExtensions) are also considered sealed, since there are no properties that violate the constraint.

Type Parameters

T extends object
The type of the input value.

Parameters

o: T
The value to be checked.

Returns o is T

true if the value is a sealed object, otherwise false.

Remarks

A sealed object is an object whose properties:

cannot be added or removed
cannot be reconfigured

However, existing property values may still be changed if they are writable.

This function only returns true for actual object values. It avoids the coercion behavior of Object.isSealed, which returns true for all non-object (primitive) values.

In contrast, Object.isSealed treats primitives such as null, undefined, numbers, and strings as already sealed, which can lead to misleading results when performing runtime validation.

Value	Result	`Object.isSealed()`
`Object.freeze( {} )`	`true`	`true`
`Object.freeze( [] )`	`true`	`true`
`Object.seal( {} )`	`true`	`true`
`Object.seal( [] )`	`true`	`true`
`{}`	`false`	`false`
`[]`	`false`	`false`
`new Date()`	`false`	`false`
`null`	`false`	`true`
`undefined`	`false`	`true`
`123`	`false`	`true`
`'text'`	`false`	`true`

Since

1.0