The TypeScript 5.3 Feature They Didn't Tell You About
TypeScript 5.3 introduces relaxed rules around readonly arrays and improvements in const type parameters.
TypeScript 5.2 introduces a new keyword - using
- that you can use to dispose of anything with a Symbol.dispose
function when it leaves scope.
typescript
{const getResource = () => {return {[Symbol.dispose]: () => {console.log('Hooray!')}}}using resource = getResource();} // 'Hooray!' logged to console
This is based on the TC39 proposal, which recently reached Stage 3 (out of 4) in its progress to JavaScript. This means that it's ready to be tested by early adopters.
using
will be extremely useful for managing resources like file handles, database connections, and more.
Symbol.dispose
Symbol.dispose
is a new global symbol in JavaScript. Anything with a function assigned to Symbol.dispose
will be considered a 'resource' - "an object with a specific lifetime" - and can be used with the using
keyword.
typescript
const resource = {[Symbol.dispose]: () => {console.log("Hooray!");},};
await using
You can also use Symbol.asyncDispose
and await using
to handle resources which need to be disposed asynchronously.
typescript
const getResource = () => ({[Symbol.asyncDispose]: async () => {await someAsyncFunc();},});{await using resource = getResource();}
This will await the Symbol.asyncDispose
function before continuing.
This will be useful for resources such as database connections, where you want to ensure that the connection is closed before the program continues.
Accessing the file system via file handlers in node could be a lot easier with using
.
Without using
:
typescript
import { open } from "node:fs/promises";let filehandle;try {filehandle = await open("thefile.txt", "r");} finally {await filehandle?.close();}
With using
:
typescript
import { open } from "node:fs/promises";const getFileHandle = async (path: string) => {const filehandle = await open(path, "r");return {filehandle,[Symbol.asyncDispose]: async () => {await filehandle.close();},};};{await using file = await getFileHandle("thefile.txt");// Do stuff with file.filehandle} // Automatically disposed!
Managing database connections is a common use case for using
in C#.
Without using
:
typescript
const connection = await getDb();try {// Do stuff with connection} finally {await connection.close();}
With using
:
typescript
const getConnection = async () => {const connection = await getDb();return {connection,[Symbol.asyncDispose]: async () => {await connection.close();},};};{await using db = await getConnection();// Do stuff with db.connection} // Automatically closed!
Share this article with your friends
TypeScript 5.3 introduces relaxed rules around readonly arrays and improvements in const type parameters.
Learn how to provide a TypeScript playground when asking for help with your TypeScript questions, making it easier for others to assist you.
Learn how to work with events in React and TypeScript, from onClick to onSubmit.
A step-by-step guide on setting up ESBuild to bundle a Node application.
When using '--moduleResolution' with the option 'nodenext', it is necessary to add explicit file extensions to relative import paths in EcmaScript imports.
Learn how to add TypeScript to your existing React project in a few simple steps.