Node.js - REPL (Read Eval Print Loop)

Johan H. Guzman Gil — Tue, 07 Apr 2020 20:07:29 +0000

Before start with this post, I would like to explain what is node.js.

Node.js is not entirely true a JavaScript that runs on the backend, it is a wrapper of a VM who is running the JavaScript V8 engine,
knowing that, node.js come with a lot API that we can use in conjunction with JavaScript but I will leave that topic for another post.

Now let talk about REPL.

REPL it is a node API that let us to write or test script in the terminal.
To enter in to the REPL mode we need to type node in the terminal.

> node
>

After that are going to see an empty > its mean that we are into the REPL mode, now we can have fun and write some code.

> console.log('hello REPL') // => hello REPL

REPL let us write an inline code by default, if we want to write a multiple line of code

> function printHelloAliens() {

then left { open and press enter you will see the next line with ...

> function printHelloAliens() {
> ...

it means that we are ready to continue with the function content, add a console.log('hello aliens') and press enter and you will see the next line with ...

> function printHelloAliens() {
> ...console.log('hello aliens');
> ...

to finish with the function content with need to add }

> function printHelloAliens() {
> ...console.log('hello aliens');
> ...}
> undefined

after press enter the terminal will print undefined;
if you want to test your function just invoke the function, in my case is printHelloAliens()

> function printHelloAliens() {
> ...console.log('hello aliens');
> ...}
> undefined
> printHelloAliens() // => hello aliens

they are a lot of options, if you want to know it just type .help

> .help
.break    Sometimes you get stuck, this gets you out
.clear    Alias for .break
.editor   Enter editor mode
.exit     Exit the repl
.help     Print this help message
.load     Load JS from a file into the REPL session
.save     Save all evaluated commands in this REPL session to a file

for this tutorial we're only going to see the .editor command.
The .editor command allow you to write a multiple line of code more easy.

> .editor
// Entering editor mode (^D to finish, ^C to cancel)

Now with can write a multiple line of code with just pressing enter

// Entering editor mode (^D to finish, ^C to cancel)
function printMyName() {
 console.log('Johan Guzman')
}
> undefined

For close the editor mode type CTRL + d and the terminal will print undefined;

if you want to test your function just invoke the function, in my case is
printMyName()

// Entering editor mode (^D to finish, ^C to cancel)
function printMyName() {
 console.log('Johan Guzman')
}
> undefined
> printMyName() // => Johan Guzman

The REPL is very helpful to test a node or JavaScript code without open the editor or write a little program.

JavaScript - Global variable

Johan H. Guzman Gil — Sat, 04 Apr 2020 03:31:31 +0000

Hi, in this episode I will talk about Global variable in JavaScript.

Let's write the next code:

function print(text) {
 var helloWorld = text;
 console.log(helloWorld);
}

print('hello my aliens') // => hello my aliens
console.log(helloWorld) // => error: Uncaught ReferenceError: helloWorld is not defined

The program is going to print the variable helloWorld inside the function but outside the function it will break the code

If we want to have access to the variable hellowWorld outside the function print we need to remove the reserved word var and our code will look like this:

function print(text) {
 helloWorld = text;
 console.log(helloWorld);
}

print('hello my aliens') // => hello my aliens
console.log(helloWorld) // => hello my aliens

And we will see two identical messages.

But the big question is, WHY THIS HAPPENS?
When JavaScript are compiling, and he found the word helloWorld without the reserved word var, it will put the variable helloWorld on top of the file because it thinks that you are going to use it below.

Your compiled code will look like this:

var helloWorld;

function print(text) {
 helloWorld = text;
 console.log(helloWorld);
}

print('hello my aliens') // => hello my aliens
console.log(helloWorld) // => hello my aliens

This method of using the global variable will work but is not recommended. There are multiple ways to have a global variable, but the complexity of the code will determine which one is the correct one. Some example of these are use a session store, create a function that return the value, enums, or use a dictionary

DEV Community: Johan H. Guzman Gil

Node.js - REPL (Read Eval Print Loop)

JavaScript - Global variable