# Deno Node.js compatibility

This module is meant to have a compatibility layer for the
[Node.js standard library](https://nodejs.org/docs/latest/api/).

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

## Supported modules

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

* [x] 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.

```ts
import { createRequire } from "node:module";

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 and running tests

See [tools/node_compat/README.md](../../../tools/node_compat/README.md).

### Best practices

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

```ts, ignore
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:

```ts, ignore
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:

```ts, ignore
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:

```ts, ignore
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](../../../tools/node_compat/TODO.md).