defer

Creates a Disposable object that runs a callback when its scope exits.

using cleanup = defer(callback);

Usage

defer(callback)

Use defer when you want cleanup code to run automatically at the end of a scope. Declare the returned object with a using declaration, and the callback runs when the enclosing block exits — even if it exits with an error.

import { defer } from 'es-toolkit/util';

function processFile() {
  const file = openFile('data.txt');
  using cleanup = defer(() => file.close());

  // The file is closed automatically when this function returns or throws.
  return file.read();
}

When several using declarations appear in the same scope, their callbacks run in reverse declaration order, like a stack.

import { defer } from 'es-toolkit/util';

function run() {
  using first = defer(() => console.log('first'));
  using second = defer(() => console.log('second'));
  console.log('body');
}

run();
// Logs 'body', 'second', then 'first'.

using declarations require TypeScript 5.2 or later, and a runtime that supports Explicit Resource Management, such as Node.js 24 or the latest browsers. In older environments, you can transpile the syntax and polyfill Symbol.dispose.

::: info Use deferAsync for asynchronous cleanup

This function runs the callback synchronously. If your cleanup code is asynchronous and should be awaited, use the deferAsync function with await using instead.

:::

Parameters

• callback (() => void): The cleanup function to run when the object is disposed.

Returns

(Disposable): An object that runs callback when it is disposed.