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# Deno Node compatibility
This module is meant to have a compatibility layer for the
[NodeJS standard library](https://nodejs.org/docs/latest-v12.x/api/).
**Warning**: Any function of this module should not be referred anywhere in the
deno standard library as it's a compatibility module.
## Supported Builtins
- [ ] assert
- [x] buffer
- [ ] child_process
- [ ] cluster
- [ ] console
- [ ] crypto
- [ ] dgram
- [ ] dns
- [x] events
- [x] fs _partly_
- [ ] http
- [ ] http2
- [ ] https
- [x] module
- [ ] net
- [x] os _partly_
- [x] path
- [ ] perf_hooks
- [x] process _partly_
- [x] querystring
- [ ] readline
- [ ] repl
- [ ] stream
- [ ] string_decoder
- [ ] sys
- [x] timers
- [ ] tls
- [ ] tty
- [ ] url
- [x] util _partly_
- [ ] ~~v8~~ _can't implement_
- [ ] vm
- [ ] worker_threads
- [ ] zlib
* [x] node globals _partly_
### Deprecated
These builtins are deprecated in NodeJS v13 and will probably not be polyfilled:
- constants
- domain
- freelist
- punycode
### Experimental
These builtins are experimental in NodeJS v13 and will not be polyfilled until
they are stable:
- async_hooks
- inspector
- policies
- report
- trace_events
- wasi
## CommonJS Module Loading
`createRequire(...)` is provided to create a `require` function for loading CJS
modules. It also sets supported globals.
```ts
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
When converting from promise-based to callback-based APIs, the most obvious way
is like this:
```ts
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
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
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
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.
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