Deno 1.40 Release Highlights

Deno has reached version 1.40 and this is one of the coolest versions yet! First, because it includes the addition of the Temporal API that I’ve been talking about here since 2020! And I think it’s the first runtime to actually implement this functionality!

Beyond that, a series of other features that I’ll go through here 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 implement this API completely.

It’s still behind an --unstable-temporal flag, but you can already use it the way the official documentation describes! If you run a terminal with deno --unstable-temporal, you can already do some tests:

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console.log(Temporal.Now.instant()) // data e hora de hoje
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console.log(Temporal.Now.zonedDateTimeISO()) // equivalente ao 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 happen. Since the advent of ECMAScript modules, the entire way module resolution works has changed, which means that getting the name of a file or the directory where it exists is no longer as simple to find.

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

But with ESM we can’t use them because they’re not set at the beginning of the application, so we have to get the module’s URL with import.meta.url, which returns a fileURL in the pattern file://path, 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 very annoying! Especially if you had to deal with paths constantly, so two proposals were added to support both the file and 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 going to have native support for decorators, a proposal that’s in final stage and should arrive in browsers soon! After more than 5 years of waiting, the new proposal combines all the previous proposals into one.

A classic example we can give is @trace, which is used to debug any function by logging to the console before and after 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 feature. 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 allowed us to 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 have just one module in the import map and be able to 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 don’t really like using import maps and prefer the approach of a deps.ts file

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 occurs in a promise that has already been rejected (when you have an unused catch)
• Support for windows with WebGPU
• Support for new native Node.js APIs
• Better messages in deno lint and deno doc