Tutorial: Synchronous and Blocking JavaScript
This is the beginning of an introduction into the blocking nature of JavaScript. In the process, you’ll also be doing more exploring of various Node modules, like fs
.
This is required reading for the Code Chrysalis Immersive Bootcamp’s precourse — a series of assignments, projects, assessments, and work that all students must successfully complete remotely before embarking on our full-stack software engineering course.
Before You Begin
- You will need to have Node installed on your computer. A simple way of describing Node is that it is a program that will allow you to run JavaScript on your computer (rather than just in the browser). This means that you will be able to control, among other things, your file system! If you are new to Node, please check out Node School to get started.
- This is a hands-on tutorial, so you will need to follow along and play around with the code yourself.
- Learning to look through and read documentation is a very important skill for software engineers. Please practice looking through the NodeJS documentation as you are going through this tutorial. Please be cognizant of the version of Node that you have and the documentation version that you are looking at.
Objectives
- Gain familiarity with using
fs
- Use
fs.readFileSync
to read files on local system - Use the words synchronous and blocking to describe certain aspects of JavaScript
Synchronous & Blocking
Let’s create a function:
console.log('1');
function hello() {
console.log('hello');
}
console.log('2');
hello();
console.log('3');
Can you predict the output?
Answer:
1
2
hello
3
What if hello
was instead a long-running operation, though? Like a database lookup or HTTP request? Don't worry about how it's implemented, just supposed that if it were, then hello
would take a lot longer to complete.
Does this mean that we can’t call the last console.log
until it's complete? With JavaScript, yes. That's because JavaScript can only do one thing at a time (advanced: single-threaded). This would be, what we call, a synchronous operation.
With a synchronous operation, if something takes a long time, then that function would block the rest of the code from running until the operation is completed.
On a browser, that can mean an unresponsive web page.
On a server, that can mean requests would stop being processed.
Switching gears a little bit…
Introducing fs
Unlike the browser, where everything is global, in Node there are only two global things: a module
object, and a require()
function.
We’ll get into module
later, for now, let's require()
fs:
const fs = require('fs');
console.log(fs);
What is fs
? There are two ways to find out:
- Check the docs
console.log()
it!
We see a lot of file
related methods, so one could deduce that it stands for "file system".
In your current directory, run the following command:
echo 'hello world' >> index.js
Then run the following in Node:
const fs = require('fs');const result = fs.readFileSync('index.js', 'utf8');
console.log(result);
What is the result? What happens if you don’t include ‘utf8’? What is ‘utf8’?
Blocking Behavior
In our above example, take note that the console.log
does not run until we are done reading the file. Try modifying index.js
so that it is a huge chunk of code. Does that change the order?
No.
This is because fs.readFileSync
is exacty as its name suggests---it is a synchronous method and therefore, blocking. No matter how large the file, our JavaScript will wait until fs.readFileSync
is completely done with reading it.
Let’s check this out with regular JavaScript.
Blocking Behavior with Higher Order Functions
Let’s create a higher order function now and provide hello
to it as a callback.
console.log('1');
function hello() {
console.log('hello');
};
console.log('2');
function invokeNow(action) {
console.log('3');
action();
}
console.log('4');
invokeNow(hello)
console.log('5');
Can you predict the output?
Answer:
1
2
4
3
hello
5
invokeNow(hello)
is still exhibiting blocking behavior. We have to wait until all of that is done before we print 5.
Challenges
- Can you use
fs.readFileSync()
to read non-JavaScript files? Like a JSON file or a text (.txt) file? How about an html file? Try it. - What are other options besides ‘utf8’?
- The opposite of synchronous is called asynchronous. What do you think asynchronous means?
- Explore
fs.readFile
. This is the asynchronous version offs.readFileSync
. How do you use it? What does it do differently?
Resources
Here are a couple of resources. These are not the only resources out on the internet, so please look around and find other resources to supplement as needed.
- Blocking vs non-blocking JavaScript — Node Documentation
- What every programmer should know about Synchronous vs. Asynchronous Code
What about Asynchronous Javascript? 👉 Read Tutorial: Asynchronous JavaScript — Callbacks, Promises, and Async/Await
Code Chrysalis is a Tokyo-based 🗼 coding school providing a full-time and part-time programming courses in English and Japanese. Join us in-person or take our classes remotely. See why we are an industry leader in technical education in Japan 🗾.
We also put on free workshops and events for the community, so follow us for the latest! To find out more about us, please go to our website.
Follow us on…
YouTube — https://www.youtube.com/c/codechrysalis
Medium Blog — https://medium.com/code-chrysalis
Note Blog — https://note.com/codechrysalis
Instagram — https://www.instagram.com/codechrysalis
Facebook — https://www.facebook.com/codechrysalis
Twitter — https://www.twitter.com/codechrysalis
日本語版 Twitter — https://www.twitter.com/codechrysalisJP
LinkedIn — https://www.linkedin.com/school/code-chrysalis/