FinalizationRegistry.prototype.register()
Baseline
Widely available
*
This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2021.
* Some parts of this feature may have varying levels of support.
The register() method of FinalizationRegistry instances registers a value with this FinalizationRegistry so that if the value is garbage-collected, the registry's callback may get called.
Syntax
register(target, heldValue)
register(target, heldValue, unregisterToken)
Parameters
target-
The target value to register.
heldValue-
The value to pass to the finalizer for this
target. This cannot be thetargetitself but can be anything else, including functions and primitives. unregisterTokenOptional-
A token that may be used with the
unregistermethod later to unregister the target value. If provided (and notundefined), this must be an object or a non-registered symbol. If not provided, the target cannot be unregistered.
Return value
None (undefined).
Exceptions
TypeError-
Thrown in one of the following cases:
targetis not an object or a non-registered symbol (object as opposed to primitives; functions are objects as well)targetis the same asheldValue(target === heldValue)unregisterTokenis not an object or a non-registered symbol
Description
See the Avoid where possible
and Notes on cleanup callbacks
sections of the FinalizationRegistry page for important caveats.
Examples
Using register
The following registers the value referenced by target,
passing in the held value "some value" and passing the target itself
as the unregistration token:
registry.register(target, "some value", target);
The following registers the value referenced by target,
passing in another object as the held value, and not passing in any unregistration token
(which means target can't be unregistered):
registry.register(target, { useful: "info about target" });
Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-finalization-registry.prototype.register |
Browser compatibility
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