FinalizationRegistry

`FinalizationRegistry.prototype.register`

Registers a target object with the FinalizationRegistry so that the cleanup callback is called when the target is garbage collected

Syntax

JavaScript

registry.register(target, heldValue, unregisterToken?)

Parameters

Parameter	Type	Description
target	object	The target object to register
heldValue	any	The value passed to the cleanup callback
unregisterToken	object	An optional token to use for unregistering

Return Value

undefined

Examples

Basic Usage

const registry = new FinalizationRegistry((val: string) => console.log('Cleaned:', val))
const obj = { x: 1 }
registry.register(obj, 'object-x')

Practical Example

const registry = new FinalizationRegistry((id: string) => {
  console.log(`Releasing handle: ${id}`)
})
const token = {}
const resource = { handle: 'abc' }
registry.register(resource, 'abc', token)

Advanced Usage

class ResourceManager {
  private registry = new FinalizationRegistry<number>(this.cleanup.bind(this))
  private resources = new Map<number, { handle: number }>()
  acquire(id: number, resource: object) {
    this.resources.set(id, { handle: id })
    this.registry.register(resource, id)
  }
  private cleanup(id: number) {
    this.resources.delete(id)
  }
}

Understanding FinalizationRegistry.prototype.register

The FinalizationRegistry.prototype.register method in JavaScript registers a target object with the FinalizationRegistry so that the cleanup callback is called when the target is garbage collected. It belongs to the FinalizationRegistry object and is one of the most widely used methods for working with finalizationregistry values in modern JavaScript and TypeScript applications.

The method signature is registry.register(target, heldValue, unregisterToken?). It accepts 3 parameters: target, heldValue, unregisterToken. When called, it returns undefined. Understanding when and how to use register() helps you write more expressive, readable code.

Common use cases for FinalizationRegistry.prototype.register include data transformation, input validation, API response processing, and building reusable utility functions. It works well alongside related methods like finalizationregistry-constructor, finalizationregistry-unregister, weakref-constructor, enabling you to chain operations together for complex data manipulation pipelines.

Supported in Chrome 84+, Firefox 79+, Safari 14.1+, Edge 84+, Node.js 14.6+.