What's New in Deno 1.40

Deno hit version 1.40 and this is one of the coolest releases of all! First, because it added the Temporal API that I’ve been talking about here since 2020! And I think it’s the first runtime to actually implement this feature!

Beyond that, a whole bunch of other updates I’ll walk through one by one.

Temporal API#

The Temporal API is the new way to handle dates in JavaScript. I talked a lot about it in this article and showed all the details, so I won’t go into too much detail here, but essentially, Deno is the first runtime to fully implement this API.

It’s still behind an --unstable-temporal flag but you can use it exactly as the official docs show! If you run a terminal with deno --unstable-temporal, you can already run some tests:

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console.log(Temporal.Now.instant()) // today's date and time
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console.log(Temporal.Now.zonedDateTimeISO()) // equivalent to toISOString
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const birthday = Temporal.PlainMonthDay.from("12-15");
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const birthdayIn2030 = birthday.toPlainDate({ year: 2030 });
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console.log(birthdayIn2030.toString()); // 2030-12-15

import.meta.filename and dirname#

This was something I really wanted to see happen. Since the advent of ECMAScript modules, the whole module resolution changed, which means getting the name of a file or the directory where it exists is no longer that simple to fetch.

In Node, we have two “magic” variables called __dirname and __filename that return the directory path and the file you’re running respectively.

But with ESM we can’t use them because they aren’t set at the beginning of the application, so we have to get the module URL with import.meta.url, which returns a fileURL in the file://path format, and then we have to use dirname on that URL, but only after converting that URL to a path!

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import { fileURLToPath } from 'node:url'
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import { dirname } from 'node:path'
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const fileurl = import.meta.url
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const path = fileURLToPath(fileurl)
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const dir = dirname(path)
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const filename = path.split('/').pop()
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console.log({ fileurl, path, dir, filename })
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/*
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{
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  fileurl: "file:///Users/lucas/repos/deno/teste.ts",
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  path: "/Users/lucas/repos/deno/teste.ts",
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  dir: "/Users/lucas/repos/deno",
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  filename: "teste.ts"
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}
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*/

This was really annoying! Especially if you had to deal with paths constantly, so two proposals were added to support both the file and the folder, called import.meta.filename and import.meta.dirname. Now you can replace all of that with:

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console.log(import.meta.dirname) // /Users/lucas/repos/deno
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console.log(import.meta.filename) // /Users/lucas/repos/deno/teste.ts

Decorators#

Finally we’re getting native support for decorators, a proposal in its final stage and should arrive in browsers soon! After more than 5 years of waiting, the new proposal brings together all the previous proposals into one.

A classic example we can give is the @trace that’s used to debug any function by putting a console before and after the execution:

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function trace(fn: any, ctx: ClassMethodDecoratorContext) {
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  return function (...args: unknown[]) {
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    console.log("ENTERED", ctx.name);
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    const v = fn(...args);
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    console.log("EXITED", ctx.name);
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    return v;
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  };
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}
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class App {
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  @trace
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  static start() {
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    console.log("Hello World!");
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  }
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}
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App.start();

Simpler imports#

This is another interesting update. In the past, when we had import maps with modules that had subdirectories, we had to define two different imports:

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{
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  "imports": {
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    "preact": "npm:preact@10.5.13",
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    "preact/": "npm:/preact@10.5.13/"
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  }
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}

This let us import both the top-level preact with import preact from 'preact', and also submodules like import hooks from 'preact/hooks'.

Now the simplification works so you can just have one module in the import map and import everything from the same root with just one map:

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{
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  "imports": {
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    "preact": "npm:preact@10.5.13"
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  }
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}

Although I’m not a big fan of using import maps and prefer the deps.ts file approach

Other changes#

• A series of APIs and commands are being deprecated, especially deno.run and deno.serveHttp
• Support for rejectionHandled, an event that fires whenever an exception happens in a promise that’s already been rejected (when you have an unused catch)
• Support for windows with WebGPU
• Support for new Node.js native APIs
• Better messages in deno lint and deno doc