learnyounode/exercises/my_first_async_io/problem.md

Learn You The Node.js For Much Win! An intro to Node.js via a set of self-guided workshops.

Write a program that uses a single **asynchronous** filesystem operation to read a file and print the number of newlines it contains to the console (stdout), similar to running `cat file | wc -l`.

The full path to the file to read will be provided as the first command-line argument.

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# HINTS

The solution to this problem is *almost* the same as the previous problem except you must now do it **the Node.js way**: asynchronous.

Instead of `fs.readFileSync()` you will want to use `fs.readFile()` and instead of using the return value of this method you need to collect the value from a callback function that you pass in as the second argument. To learn more about callbacks, check out: https://github.com/maxogden/art-of-node#callbacks.

Remember that idiomatic Node.js callbacks normally have the signature:

```js
function callback (err, data) { /* ... */ }
```

so you can check if an error occurred by checking whether the first argument is truthy. If there is no error, you should have your `Buffer` object as the second argument. As with `readFile()`, you can supply 'utf8' as the second argument and put the callback as the third argument and you will get a `String` instead of a `Buffer`.

Documentation on the `fs` module can be found by pointing your browser here:
  {rootdir:/docs-nodejs/fs.html}