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HomeBlogWeb DevelopmentNode.js REPL: Getting Started
Node.js is an Open-Source and Cross-platform JavaScript runtime environment built on V8 JavaScript engine – the core of google chrome. It is used to build high-performing and scalable applications and is an event-driven non-blocking I/O model.
Node.js, when installed, comes with REPL environment which stands for READ, EVAL, PRINT, and LOOP which is similar to a computer environment like a command prompt or Shell. This environment interacts with the user writing or debugging through the outputs of his commands/expressions.
To switch to this environment, open the terminal and enter ‘node’ text and press ‘Enter’ key. Let’s follow along with each keyword.
In terminal:
machine-name ~ % node Welcome to Node.js v14.16.1. Type ".help" for more information. >
As you can see from the above pic, REPL is demarcated by symbol ‘>’ once it is started. As a user, we can input any command or expression to get REPL.
Note: Going forward we would be switching between node.js terminal and chrome console based on convenience, but both would mean the same.
Now let's start giving a few basic commands and understand how REPL works
Invoke the REPL by entering the node and pressing the ‘enter’ key. You should be seeing the symbol ‘>’ ready to take input for evaluation.
In browser console:
> var greet = "Welcome to Node.Js REPL" <. undefined > greet <. 'Welcome to Node.Js REPL' > greet = "Good to see you here!" <. 'Good to see you here!' > greet <. 'Good to see you here!' >
In the first line, we created a variable named greet with a value of ‘Welcome to Node.Js REPL’. REPL responded with undefined as the expression doesn’t return anything.
Next, we entered the variable name ‘greet’ and REPL responded with the value stored in previous execution i.e., Welcome to Node.Js REPL.
Post this, we tried reset the value of ‘greet’ variable with different text and on click on enter it responded with the new value in the ‘greet’. To recheck the value is set correctly, we re-entered the greet and REPL responded with the new value set i.e. ‘Good to see you here!’.
Note: Output of the command is demarcated by symbol ‘<’ with dot in chrome and without symbol in terminal.
Let’s take another example where we want to perform some arithmetical operations in REPL.
In browser console:
> 1 + 1 // addition <. 2 > 2 - 1 //subtraction <. 1 > 2 * 2 // multiplication <. 4 > 6 / 3 // division <. 2 > 10 % 3 // remainder <. 1 > 3 * 4 - 2 / 2 + 1 // follows BODMAS principle <. 12 >
In the above example we are able to perform different arithmetic operations individually and together. And from the last example, it is evident that REPL internally follows BODMAS principle for performing operations.
Node.js also comes with a library called Math which is used to perform some more complex operations. Let’s see few of them which are used more often in action.
Examples of Math methods in the browser console:
> Math.floor(4/3) // round to lower integer <. 1 > Math.ceil(4/3) // round to closest higher integer <. 2 > Math.round(4.6) // rounds to higher if decimal is greater than or equal to .5 otherwise to lower integer <. 5 > Math.sqrt(4) // Square root <. 2 > Math.pow(2, 3) // 2 raised to the power of 3. <. 8 >
Till now we were using simple one-line expressions. Let’s try writing functions and get them executed as well. Let’s write a function which gives random number for a given maximum say 10, 100, 1000.
> function getRandomNumber(max) {
let result = Math.random() * max;
return Math.floor(result);
}
<. undefined
> getRandomNumber(10)
<. 1
> getRandomNumber(10)
<. 7
> getRandomNumber(100)
<. 59
> getRandomNumber(1000)
<. 431
>An important observation here is that after the first line curly bracket REPL didn’t evaluate the statement/expression. Instead, it went on the input mode to take more user text. After closing the curly bracket, it stored the function for future evaluation.
Note: Math.random() returns a random decimal value within the range of 0 to 1.
We can also execute loops in REPL. As we observed for functions, REPL will wait for the closing curly brace and then evaluates. Let’s take a for loop to demonstrate it.
> for (var i = 0; i < 100; i++) {
var sqrt = Math.sqrt(i);
if (Math.floor(sqrt) === Math.ceil(sqrt)) {
console.log(i);
}
}
0
1
4
9
16
25
36
49
64
81
<. undefined
>Note that the output is undefined but during execution/evaluation of for-loop we added the console.log to display the numbers which have prefect square roots with integer values.
Also, REPL didn’t execute the expression after the first closing curly brace as REPL understands it is for the IF statement and not for the FOR loop. So, it tracks the number of open and closed curly brackets.
In node.js terminal, we have a few other options which are listed down and can be obtained by writing ‘.help’ and pressing the ‘ENTER’ key.
COMMAND | DESCRIPTION |
| .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 |
From the above list, we have already used a few of them like .exit and .help. Let’s look at the other options.
.break - When in the process of inputting a multi-line expression like writing a for loop, enter the .break command (or press Ctrl+C) to abort further input or processing of that expression.
.Save - Helps to save all the operations done to the file for future reference.
.load - helps to load the file into the current REPL session.
Node.js also comes up with a REPL module that is available as standalone or includible in other applications. All the functionalities we discussed above are available in this module as well. The respective input and output are defaulted to Process.stdin and Process.stdout can be configured to any other Node.js stream.
Since it is exposed as a module it has access to the internal core modules and load them in REPL environment.
REPL uses the domain module for all uncaught exceptions in their respective session. And the effect of this is that the uncaught exception only emits the ‘uncaughtException’ event and a listener to this event in other node.js results in ERR_INVALID_REPL_INPUT.
Similarly, if we use process.setUncaughtExceptionCaptureCallback() then it will show an ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE error.
Await keyword is supported and enabled. Let’s look at an example.
> async function Sample() {
... console.log(await Promise.resolve('test'));
... }
undefined
> Sample()
Promise { <pending> }
> testAn REPL can be exposed from node.js application with the help of repl module.
import repl from 'repl';
repl.start({
prompt: 'Node.js via stdin:',
input: process.stdin,
output: process.stdout
});Output:
With the help of environment variables, REPL behavior can be customized.
Single/Multiple REPL instances can be created and run in an instance of Node.js which can be shared via global object.
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The following example provides a REPL instance on stdin of a TCP socket and the same could be created in different Unix socket or same socket with a different port.
Steps:
server.js file:
import net from 'net';
import repl from 'repl';
net.createServer(function (socket) {
var r = repl.start({
prompt: 'Node.js via TCP socket> '
, input: socket
, output: socket
, terminal: true
, useGlobal: false
})
r.on('exit', function () {
socket.end()
})
}).listen(5001);client.js file:
import net from 'net';
var sock = net.connect(5001)
process.stdin.pipe(sock)
sock.pipe(process.stdout)
sock.on('connect', function () {
process.stdin.resume();
process.stdin.setRawMode(true)
})Output:
REPL is an interactive environment that comes with installing node.js, helps to execute JS code without saving or writing to a file. Understanding this is important to anyone who wants to work with Node. Try what you have read here starting from the basic arithmetic operation, expressions to having node.js REPL over net.server.
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