README.md

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In this lesson, we are going to learn about Node.js REPL. REPL stands for Read Evaluate Print Loop, and that is basically what it does: It reads an input, evaluates it, prints the result and starts the process again. The REPL could be very helpful to quickly test simple JavaScript code.

Script

Let's try it out

Let's get started with Node.js REPL by invoking the node command in the terminal, with no arguments.

> node
> 

Here, Node expects you to type an expression and it will print the result of that expression. For example:

> node
> 1 + 1
2
> x = 1 + 2
3
> x * 2
6

Next, try with a console.log

> console.log('Hello World')
Hello World
undefined

Here, Hello World is the output we told the console to print, then we got undefined which is the return value of running console.log(). Here Node reads this line, evaluates it, prints the result and then went back to waiting for more lines of code.

Multiline code

In some cases, you might need to evaluate a code block consisting multiple lines, for example a function:

function printLog() {
....

The REPL is quite smart to determine that you have not completed the code block, it will go into a multi-line mode for you to type in more code.

function printLog() {
  console.log('Multiline function check')
}
undefined

Node will get out of the multi-line mode, and print undefined since there is no value returned.

Autocompletion

Another cool trick often forgotten is the autocompletion key. The node REPL autocompletes commands when you hit the <tab> key.

> str = "Hello, this is a string"
Hello, this is a string
> str. (press <Tab> key to get the suggestions)

The underscore character _

In the Node.js REPL you can reference the last value using the underscore character _:

> "John Doe"
John Doe
> `My name is: ${_}`
My name is: John Doe

Few important Node.js REPL commands

There are some special commands that you can send to the REPL. These commands start with a dot:

.exit command

The .exit command finishes the REPL session. It’s the same thing as typing ctrl-d.

.save and .load commands

.save and .load are two useful commands when working with the REPL. .save allows you to save your current REPL session. The output file is a list of every expression you’ve run in that session:

> arr = [1,2,3,4]
[1,2,3,4]
> str = "Hello world"
"Hello world"
> .save mysession
Session saved to:mysession
> .exit

Once you are out of REPL session, you can print the session values:

> cat mysession
arr = [1,2,3,4]
str = "Hello world"

Now you can load the session back into the REPL with the .load command:

> node
> .load myssion
arr = [1,2,3,4]
str = "Hello world"
> str
Hello world
> arr
[1,2,3,4]

REPL history

By default, the expression history is persistent. Every expression you evaluate in the REPL will be stored in a .node_repl_history in your $HOME directory.

That's all for this lesson, see you in the next one.