Your Disk

We won't have to deal with most of the physical components of the computer when writing our programs, but we should understand about what the disk is and does.

We haven't learned to store anything yet

The result of our programs isn't capable of storing any data yet- one of the most important features of any computer system. We need to be able to build a way to store things on our server, and manipulate them.

(You may have used localStorage in your project, but it's just a hack that works around the official spec of how to build a web application.)

The only official way that we've seen to store data is when you press Command-S on your script and HTML files. Your programs don't have the ability to save anything that the user does.

We can write node.js programs that store data to our disk.

We will be using the jsonfile node package to write plain JSON text to our disk. This will come in handy later when we need to store info in our web applications.

Begin in a new directory

mkdir jsondisk
cd jsondisk

Make a node project

npm init

Add the library

npm install jsonfile

Add your js file

touch index.js

Edit your file with this text:

sublime index.js

const jsonfile = require('jsonfile');

const file = 'data.json'

jsonfile.readFile(file, (err, obj) => {

  console.log(obj)
})

Run the file.

node index.js

Nothing will happen because we specified a file that doesn't exist.

Create the file.

touch data.json

Run the file.

node index.js

Nothing will happen because the file is empty.

Put something in the json file.

{
  "bannana":"monkey",
  "coconut":"sloth"
}

Note you don't need to assign it to a variable.

Run the file.

node index.js

Writing to the file:

const jsonfile = require('jsonfile');

const file = 'data.json'

const obj = {
  "hello" : "banana",
  "goodbye" : "banana"
};

jsonfile.writeFile(file, obj, (err) => {
  console.log(err)
});

Reading and Writing to the file:

So far we can read all the contents of a file, and then write all the contents of a file.

In order to add something to the file, we need to read the content, then when we have all the data in the file assigned to a variable (obj in the example below), we can alter it, then write it back out.

const jsonfile = require('jsonfile');

const file = 'data.json'

jsonfile.readFile(file, (err, obj) => {

  console.log(obj);
  obj["helloworld"] = "monkey";

  jsonfile.writeFile(file, obj, (err) => {
    console.log(err)
  });
});

Pairing Exercise:

• Follow the setup instructions above.

• Create the JSON file in sublime.

• Read the JSON file.

• Check it against the JSON text in the sublime file.

• Write to the json file.

• Check it against the JSON text in the sublime file.

• Write something new to the json file.

• Check it against the JSON text in the sublime file.

• Read and then write back out to the JSON file.

• Check it against the JSON text in the sublime file.

Further

Take 2 command line arguments and write them as key and value into the JSON.