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.

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.