denoland-deno/ext/node/polyfills

Deno Node.js compatibility

This module is meant to have a compatibility layer for the Node.js standard library.

Warning: Any function of this module should not be referred anywhere in the Deno standard library as it's a compatibility module.

Supported modules

• assert
• assert/strict partly
• async_hooks partly
• buffer
• child_process partly
• cluster partly
• console partly
• constants partly
• crypto partly
• dgram partly
• diagnostics_channel partly
• dns partly
• events
• fs partly
• fs/promises partly
• http partly
• http2
• https partly
• inspector partly
• module
• net
• os partly
• path
• path/posix
• path/win32
• perf_hooks
• process partly
• punycode
• querystring
• readline
• repl partly
• stream
• stream/promises
• stream/web partly
• string_decoder
• sys
• timers
• timers/promises
• tls
• trace_events
• tty partly
• url
• util partly
• util/types partly
• v8
• vm partly
• wasi
• webcrypto
• worker_threads
• zlib

• node globals partly

Deprecated

These modules are deprecated in Node.js and will probably not be polyfilled:

• domain
• freelist

Experimental

These modules are experimental in Node.js and will not be polyfilled until they are stable:

• diagnostics_channel
• async_hooks
• policies
• trace_events
• wasi
• webcrypto

CommonJS modules loading

createRequire(...) is provided to create a require function for loading CJS modules. It also sets supported globals.

import { createRequire } from "https://deno.land/std@$STD_VERSION/node/module.ts";

const require = createRequire(import.meta.url);
// Loads native module polyfill.
const path = require("path");
// Loads extensionless module.
const cjsModule = require("./my_mod");
// Visits node_modules.
const leftPad = require("left-pad");

Contributing

Setting up the test runner

This library contains automated tests pulled directly from the Node.js repo in order ensure compatibility.

Setting up the test runner is as simple as running the node/_tools/setup.ts file, this will pull the configured tests in and then add them to the test workflow.

$ deno task node:setup

You can additionally pass the -y/-n flag to use test cache or generating tests from scratch instead of being prompted at the moment of running it.

# Will use downloaded tests instead of prompting user
$ deno run --allow-read --allow-net --allow-write node/_tools/setup.ts -y
# Will not prompt but will download and extract the tests directly
$ deno run --allow-read --allow-net --allow-write node/_tools/setup.ts -n

To run the tests you have set up, do the following:

$ deno test --allow-read --allow-run node/_tools/test.ts

If you want to run specific Node.js test files, you can use the following command

$ deno test -A node/_tools/test.ts -- <pattern-to-match>

For example, if you want to run only node/_tools/test/parallel/test-event-emitter-check-listener-leaks.js, you can use:

$ deno test -A node/_tools/test.ts -- test-event-emitter-check-listener-leaks.js

If you want to run all test files which contains event-emitter in filename, then you can use:

$ deno test -A node/_tools/test.ts -- event-emitter

The test should be passing with the latest deno, so if the test fails, try the following:

• $ deno upgrade
• $ git submodule update --init
• Use --unstable flag

To enable new tests, simply add a new entry inside node/_tools/config.json under the tests property. The structure this entries must have has to resemble a path inside https://github.com/nodejs/node/tree/main/test.

Adding a new entry under the ignore option will indicate the test runner that it should not regenerate that file from scratch the next time the setup is run, this is specially useful to keep track of files that have been manually edited to pass certain tests. However, avoid doing such manual changes to the test files, since that may cover up inconsistencies between the node library and actual node behavior.

Working with child processes ? Use DENO_NODE_COMPAT_URL

When working with child_process modules, you will have to run tests pulled from Node.js. These tests usually spawn deno child processes via the use of process.execPath. The deno executable will use its own embedded version of std modules, then you may get the impression your code is not really working as it should.

To prevent this, set DENO_NODE_COMPAT_URL with the absolute path to your deno_std repo, ending with a trailing slash:

export DENO_NODE_COMPAT_URL=$PWD/
# or
export DENO_NODE_COMPAT_URL=file:///path/to/deno_std/dir/

Then, deno will use your local copy of deno_std instead of latest version.

Best practices

When converting from promise-based to callback-based APIs, the most obvious way is like this:

promise.then((value) => callback(null, value)).catch(callback);

This has a subtle bug - if the callback throws an error, the catch statement will also catch that error, and the callback will be called twice. The correct way to do it is like this:

promise.then((value) => callback(null, value), callback);

The second parameter of then can also be used to catch errors, but only errors from the existing promise, not the new one created by the callback.

If the Deno equivalent is actually synchronous, there's a similar problem with try/catch statements:

try {
  const value = process();
  callback(null, value);
} catch (err) {
  callback(err);
}

Since the callback is called within the try block, any errors from it will be caught and call the callback again.

The correct way to do it is like this:

let err, value;
try {
  value = process();
} catch (e) {
  err = e;
}
if (err) {
  callback(err); // Make sure arguments.length === 1
} else {
  callback(null, value);
}

It's not as clean, but prevents the callback being called twice.

Remaining Tests

Node compatibility can be measured by how many native Node tests pass. If you'd like to know what you can work on, check out the list of Node tests remaining here.