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# Using the Hyperdrive FS
[Hyperdrive](https://github.com/mafintosh/hyperdrive), the core file system module in dat, exposes an API that mimics the Node fs API. This allows you to create modules that act on hyperdrive or a regular fs with the same API. We have several modules that we make use of this custom fs, such as [mirror-folder](https://github.com/mafintosh/mirror-folder).
Mirror folder can copy a regular directory to another regular directory:
```js
var mirror = require('mirror-folder')
mirror('/source', '/dest', function (err) {
console.log('mirror complete')
})
```
You can also copy a folder to an archive (this is how we do importing in Dat):
```js
var archive = hyperdrive('/dir')
mirror('/source', {name: '/', fs: archive}, function (err) {
console.log('mirror complete')
})
```
### Creating Custom FS Modules
To create a module that uses a custom fs, you can default to the regular `fs` but also accept `fs` as an argument. For example, to print a file you could write this function:
```js
function printFile(file, fs) {
if (!fs) fs = require('fs')
fs.readFile(file, 'utf-8', function (err, data) {
console.log(data)
})
}
```
Then you could use this to print a file from a regular fs:
```js
printFile('/data/hello-world.txt')
```
Or from a hyperdrive archive:
```js
var archive = hyperdrive('/data')
printFile('/hello-world.txt', archive) // pass archive as the fs!
```
## Modules!
See more examples of custom-fs modules:
* [mirror-folder](https://github.com/mafintosh/mirror-folder) - copy files from one fs to another fs (regular or custom)
* [count-files](https://github.com/joehand/count-files) - count files in regular or custom fs.
* [ftpfs](https://github.com/maxogden/ftpfs) - custom fs for FTPs
* [bagit-fs](https://github.com/joehand/bagit-fs) - custom fs module for the BagIt spec.
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