Modules In JavaScript
Tags: modules, javascript, website, import
Published: 2021-04-09 10:43:22

Modules in Javascript 
=====================

Differences between Node.js and browsers

* * * * *

### Modules in Javascript 

#### **Differences between Node.js and browsers** 

There are many differences between Node.js and browser environments, but
many of them are small and inconsequential in practice. For example, in
our *Asynchronous* lesson, we noted how [Node's
setTimeout](https://nodejs.org/api/timers.html#timers_settimeout_callback_delay_args)
has a slightly different return value from [a browser's
setTimeout](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setTimeout).
Let's go over a few notable differences between the two environments.

**Global vs Window**

In the Node.js runtime, the [global
object](https://developer.mozilla.org/en-US/docs/Glossary/Global_object)
is the object where global variables are stored. In browsers, the
[window object](https://developer.mozilla.org/en-US/docs/Web/API/Window)
is where global variables are stored. The window also includes
properties and methods that deal with drawing things on the screen like
images, links, and buttons. Node doesn't need to draw anything, and so
it does not come with such properties. This means that you can't
reference window in Node.

*Most browsers allow you to reference global but it is really the same
object as window.*

**Document**

Browsers have access to a document object that contains the HTML of a
page that will be rendered to the browser window. There is no document
in Node.

**Location**

Browsers have access to a location that contains information about the
web address being visited in the browser. There is no location in Node,
since it is not on the web.

**Require and module.exports**

Node has a predefined require function that we can use to import
installed modules like readline. We can also import and export across
our own files using require and module.exports. For example, say we had
two different files, animals.js and cat.js, that existed in the same
directory:

If we execute animals.js in Node, the program would print 'Sennacy is a
great pet!'.

Browsers don't have a notion of a file system so we cannot use require
or module.exports in the same way.

### The fs module 

Node comes with an [fs module](https://nodejs.org/api/fs.html) that
contains methods that allow us to interact with our computer's **F**ile
**S**ystem through JavaScript. No additional installations are required;
to access this module we can simply `require`{.markup--code
.markup--p-code} it. We recommend that you code along with this reading.
Let's begin with a `change-some-files.js`
script that imports the module:

```js
// change-some-files.js
```

``` js
const fs = require("fs");
```

Similar to what we saw in the `readline`
lesson, `require` will return to us a
object with many properties that will enable us to do file I/O.

***Did you know?****I/O is short for input/output. It's usage is
widespread and all the hip tech companies are using it, like.io.*

The `fs` module contains tons of
functionality! Chances are that if there is some operation you need to
perform regarding files, the `fs` module
supports it. The module also offers both synchronous and asynchronous
implementations of these methods. We prefer to not block the thread and
so we'll opt for the asynchronous flavors of these methods.

### Creating a new file 

To create a file, we can use the `writeFile`{.markup--code
.markup--p-code} method. According to the documentation, there are a few
ways to use it. The most straight forward way is:

The code a[create-a-nnew-file.js
(github.com)](https://gist.github.com/bgoonz/8898ad673bd2ecee9d93f8ec267cf213)bove
will create a new file called `foo.txt`
in the same directory as our `change-some-file.js`{.markup--code
.markup--p-code} script. It will write the string
`'Hello world!'` into that newly created
file. The third argument specifies the encoding of the characters. There
are different ways to encode characters;
[UTF-8](https://en.wikipedia.org/wiki/UTF-8) is the most common and
you'll use this in most scenarios. The fourth argument to
`writeFile` is a callback that will be
invoked when the write operation is complete. The docs indicate that if
there is an error during the operation (such as an invalid encoding
argument), an error object will be passed into the callback. This type
of error handling is quite common for asynchronous functions. Like we
are used to, since `writeFile` is
asynchronous, we need to utilize *callback chaining* if we want to
guarantee that commands occur *after* the write is complete or fails.

*Beware! If the file name specified to*`writeFile`{.markup--code
.markup--p-code}*already exists, it will completely overwrite the
contents of that file.*

We won't be using the `foo.txt` file in
the rest of this reading.

### Reading existing files {#aac1 .graf .graf--h3 .graf-after--p name="aac1"}

To explore how to read a file, we'll use VSCode to manually create a
`poetry.txt` file within the same
directory as our `change-some-file.js`
script. Be sure to create this if you are following along.

Our `poetry.txt` file will contain the
following lines:

```js 
My code fails
```

```js 
I do not know why
```

```js 
My code works
```

```js 
I do not know why
```

We can use the `readFile` method to read
the contents of this file. The method accepts very similar arguments to
`writeFile`, except that the callback may
be passed an error object and string containing the file contents. In
the snippet below, we have replaced our previous
`writeFile` code with
`readFile`:

> Running the code above would print the following in the terminal:

```js 
THE CONTENTS ARE:
```

```js 
My code fails
```

```js 
I do not know why
```

```js 
My code works
```

```jsjs
I do not know why
```

Success! From here, you can do anything you please with the data read
from the file. For example, since `data`
is a string, we could split the string on the newline character
`\n` to obtain an array of the file's
lines:

```js 
THE CONTENTS ARE:
```

```js 
[ 'My code fails',
```

```js 
'I do not know why',
```

```js
'My code works',
```

```js
'I do not know why' ]
```

```js 
The third line is My code works
```

### File I/O 

*Using the same*`poetry.txt`*file from
before:*

``` 
My code fails
```

``` 
I do not know why
```

``` 
My code works
```

``` 
I do not know why
```

Let's replace occurrences of the phrase 'do not' with the word 'should'.

We can read the contents of the file as a string, manipulate this
string, then write this new string back into the file.

We'll need to utilize callback chaining in order for this to work since
our file I/O is asynchronous:

Executing the script above will edit the `poetry.txt` file to contain:

``` 
My code fails
```

```
I should know why
```

``` 
My code works
```

``` 
I should know why
```

#### Refactor:

#### If you found this guide helpful feel free to checkout my github/gists where I host similar content: 

[bgoonz's gists · GitHub](https://gist.github.com/bgoonz)

[**bgoonz — Overview**\
*Web Developer, Electrical Engineer JavaScript | CSS | Bootstrap |
Python | React | Node.js | Express |
Sequelize…*github.com](https://github.com/bgoonz "https://github.com/bgoonz")[](https://github.com/bgoonz)

Or Checkout my personal Resource Site:

## Web Dev Resource Hub
[hi](web-dev-resource-site)

By [Bryan Guner] on [March 8,
2021](https://medium.com/p/a55333e35978).