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Unstable Feature Flags

New features of the Deno runtime are often released behind feature flags, so users can try out new APIs and features before they are finalized. Current unstable feature flags are listed on this page, and can also be found in the CLI help text by running:

deno --help

Using flags at the command line

You can enable a feature flag when you run a Deno program from the command line by passing in the flag as an option to the CLI. Here's an example of running a program with the --unstable-byonm flag enabled:

deno run --unstable-byonm main.ts

Configuring flags in deno.json

You can specify which unstable features you'd like to enable for your project using a configuration option in deno.json.

deno.json
{
"unstable": ["bare-node-builtins", "webgpu"]
}

The possible values in the unstable array are the flag names with the --unstable- prefix removed.

Configuration via environment variables

Some flags can be enabled by setting a value (any value) for an environment variable of a given name, rather than being passed as a flag or deno.json configuration option. Flags that are settable via environment variables will be noted below.

Here's an example of setting the --unstable-bare-node-builtins flag via environment variable:

export DENO_UNSTABLE_BARE_NODE_BUILTINS=true

--unstable-bare-node-builtins

Environment variable: DENO_UNSTABLE_BARE_NODE_BUILTINS

This flag enables you to import Node.js built-in modules without a node: specifier, as in the example below. You can also use this flag to enable npm packages without an npm: specifier if you are manually managing your Node.js dependencies (see byonm flag).

example.ts
import { readFileSync } from "fs";

console.log(readFileSync("deno.json", { encoding: "utf8" }));

--unstable-byonm

Environment variable: DENO_UNSTABLE_BYONM

This feature flag enables support for resolving modules from a local node_modules folder that you manage outside of Deno with npm, pnpm, or yarn. This may improve compatibility with Node.js modules that have hard requirements on the installation behavior of npm clients, or the presence of a node_modules folder.

In your Deno project folder, include a package.json file which declares your dependencies, and manage them through an npm client as you would normally. Consider a package.json with the following dependencies:

package.json
{
...
"dependencies": {
"cowsay": "^1.5.0"
}
...
}

You would install them as usual with:

npm install

Afterward, you could write code in a Deno program that looks like this:

example.ts
import cowsay from "cowsay";

console.log(cowsay.say({
text: "Hello from Deno using BYONM!",
}));

--unstable-sloppy-imports

Environment variable: DENO_UNSTABLE_SLOPPY_IMPORTS

This flag enables behavior which will infer file extensions from imports that do not include them. Normally, the import statement below would produce an error:

foo.ts
import { Example } from "./bar";
console.log(Example);
bar.ts
export const Example = "Example";

Executing the script with sloppy imports enabled will remove the error, but provide guidance that a more performant syntax should be used.

Sloppy imports will allow (but print warnings for) the following:

  • Omit file extensions from imports
  • Use incorrect file extensions (e.g. importing with a .js extension when the actual file is .ts)
  • Import a directory path, and automatically use index.js or index.ts as the import for that directory

deno compile does not support sloppy imports.

--unstable-unsafe-proto

Deno made a conscious decision to not support Object.prototype.__proto__ for security reasons. However there are still many npm packages that rely on this property to work correctly.

This flag enables this property. Note that it is not recommended to use this, but if you really need to use a package that relies on it, the escape hatch is now available to you.

--unstable-webgpu

Enable the WebGPU API in the global scope, as in the browser. Below is a simple example to get basic information about the GPU using this API:

// Try to get an adapter from the user agent.
const adapter = await navigator.gpu.requestAdapter();
if (adapter) {
// Print out some basic details about the adapter.
const adapterInfo = await adapter.requestAdapterInfo();

// On some systems this will be blank...
console.log(`Found adapter: ${adapterInfo.device}`);

// Print GPU feature list
const features = [...adapter.features.values()];
console.log(`Supported features: ${features.join(", ")}`);
} else {
console.error("No adapter found");
}

Check out this repository for more examples using WebGPU.

--unstable-broadcast-channel

Enabling this flag makes the BroadcastChannel web API available for use in the global scope, as in the browser.

--unstable-worker-options

Enable unstable Web Worker API options. Specifically, it enables you to specify permissions available to workers:

new Worker(`data:application/javascript;base64,${btoa(`postMessage("ok");`)}`, {
type: "module",
deno: {
permissions: {
read: true,
},
},
}).onmessage = ({ data }) => {
console.log(data);
};

--unstable-cron

Enabling this flag makes the Deno.cron API available on the Deno namespace.

--unstable-kv

Enabling this flag makes Deno KV APIs available in the Deno namespace.

--unstable-ffi

Enable unstable FFI APIs - learn more about FFI.

--unstable-fs

Enable unstable file system APIs in the Deno namespace. These APIs include:

--unstable-net

Enable unstable net APIs in the Deno namespace. These APIs include:

--unstable

--unstable is deprecated - use granular flags instead

The --unstable flag is no longer being used for new features, and will be removed in a future release. All unstable features that were available using this flag are now available as granular unstable flags, notably:

  • --unstable-kv
  • --unstable-cron

Please use these feature flags instead moving forward.

Before more recent Deno versions (1.38+), unstable APIs were made available all at once using the --unstable flag. Notably, Deno KV and other cloud primitive APIs are available behind this flag. To run a program with access to these unstable features, you would run your script with:

deno run --unstable your_script.ts

It is recommended that you use the granular unstable flags instead of this, the --unstable flag is now depreacted and will be removed in Deno 2.