Intro

In this section, you will be writing code that runs on the computer using node, an application that runs JavaScript code. Everything in this section is about figuring out how to use the libraries that engineers have written for you to build the application you want.

Preflight

Before you start, make sure you understand these concepts.

1. (line by line) node app.js runs a file called app.js. When a file is run, the computer will go through the code, line by line starting at the top.

   // app.js
   const fruits = []
   fruits.push('apple')
   console.log(fruits)
   

   Let's say you run the file a few times:

   node app.js
   node app.js
   node app.js
   
   // What gets printed out?
   

   • Answer

2. (async) Make sure you understand the async nature of JavaScript!

   let counter = 1
   setTimeout(() => {
     console.log(counter)
     counter = counter + 1
   }, 1)
   console.log(counter)
   // what gets printed out?
   

   • Answer

3. (Non-Primitive) Make sure you understand the nature of objects in JavaScript!

   const data = {}
   setTimeout(() => {
     const newObj = data
     console.log(newObj.name)
     newObj.name = "hello"
   }, 10)
   setTimeout(() => {
     const newObj = data
     console.log(newObj.name)
     newObj.name = 300
   }, 5)
   console.log(data.name)
   

   • Answer

Overview

First you will use a few libraries to help you learn the underlying concepts behind the internet. Once you understand these concepts, we will go over some problems that companies face when trying to scale their technology to prepare you for system design.

Previously, you learned that URL is made up of the protocol, hostname, path, and query parameters.

We can pass data into a request by using the URL's query parameters.

• Protocol - Specifies how the data is transferred and how to interpret the request. When you look at the protocol, you should have a high level idea of what the url is used for.
  • http - Runs on port 80 by default, specifies what headers are in the request and what the header means. The headers we cover in this lesson deals specifically with the http protocol.
  • https - Same as http protocol with two differences:
    • Runs on port 443 by default
    • Encrypts all of your request / response headers except request IP
• Path & query parameters: Sends data to the server in the request url. This is usually used in marketing teams to see how advertising is doing.
  • & separates the different pieces of data.
    • https://macys.com/shoes?size=4&brand=allbirds&type=outdoors
  • Sending data using path and query parameters has a few notable drawbacks:
    • Security risk: All data in the URL is exposed. Sensitive data like usernames and passwords must never be put in the query parameters. Imagine sending someone a postcard with your username and password on the postcard. Everyone can see it, it is not secure.
    • Data limit: The URL cannot hold large amounts of data. In fact, many browsers will not be able to handle URLs longer than 2048 characters. That is not a lot—only 2KB of data.
      • Imagine sending someone a postcard, there is only so much you can write before you run out of space

In this chapter you will learn other ways of sending data to the server. Everything you learn will only be applicable to the http or https protocol. The protocol is the first letters before :// in a url.

Quick overview of other protocols (Feel free to read more about these, but you are not expected to know these in your day to day role as a software engineer).

• SMTP, POP3, IMAP - Email protocols used for sending emails. Usually runs on port 25, 110, 143 respectively.
• SSH - Protocol for accessing remote machines as well as managing your git repositories. Usually runs on port 22.
• FTP - Protocol for sending files. Usually port 21.