REPL

Learn about the REPL module in Node.js.

We'll cover the following

Read Evaluate Print Loop (REPL)

As the heading suggests, REPL does exactly what it stands for. You may already have used a REPL environment before but might not have known about it. Terminals, and even the console on Google Chrome, are REPL environments. A REPL environment can be a quick and easy way to test some code. Think of it as a rough paper to run some code quickly. The REPL reads the code, evaluates it, and prints the output in the next line. This happens in a loop until the REPL exits. This allows us to experiment with the code without having to compile it. The Node.js REPL can be accessed from the command-line, or we can customize and create our own REPL environment using the repl module.

Commands and special keys

The _ variable

The default evaluator will assign the result of the most recently evaluated expression to the special variable, _ (underscore). However, if we explicitly set _ to a value, it will disable this behavior. Similarly, we can use _error to access the last error.

Commands

Commands Explanation
.break Breaks from a multiline function
.clear Resets the REPL context and breaks from a multiline function
.editor Enters edit mode
.exit Exits the current session
.help Lists down all commands
.load Loads a file to the current session
.save Saves the current session to a file

Keys

Keys Explanation
Ctrl + C Press once for a .break or twice for .exit
Ctrl + D Shortcut for .exit
TAB Pressing twice on an empty line prints global and local variables; pressing while entering input will result in an autocomplete or relevant autocomplete options

Now that we know about its commands, let’s give it a go!

The simple way

The easiest way to access a REPL environment for Node.js is to type node on the terminal, given Node.js has been installed. You don’t have to worry about that here. Just write node and give it a go.

Terminal 1
Terminal
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Using REPLServer

We can customize our REPL environment by using the REPLServer class. Importing the repl module exports repl.REPLServer. We mentioned context while explaining the clear method. The context object is associated with the REPLServer class and can be used to expose a variable to the REPL. Let’s use it with an example below.

const repl = require('repl');
const msg = 'This is our message';

repl.start('---> ').context.textMessage = msg;
Type node app.js to run the file again

Let’s see what is happening in the code above.

  • We import the repl module in line 1.
  • msg is set as a string, This is our message, in line 2.
  • We start the environment using the .start method. Whatever string we pass to this method will be the input prompt to display. Try setting it blank and see what happens.
  • We set up the context object with a new property, textMessage, and set that equal to the msg we defined above. We can see our string by typing textMessage in the REPL environment.
  • You can also start the environment without setting the textMessage by using just the start() method.

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