Learning Node, 2nd Edition

Chapter 4. Interactive Node with REPL and More on the Console

While you’re exploring the use of Node and figuring out the code for your custom module or Node application, you don’t have to type JavaScript into a file and run it with Node to test your code. Node also comes with an interactive component known as REPL, or read-eval-print loop.

REPL (pronounced “repple”) supports simplified line editing and a small set of basic commands. Whatever you type into REPL is, for the most part, processed no differently than if you had typed the JavaScript into a file and run the file using Node. You can actually use REPL to code your entire application—literally testing the application on the fly.

In this chapter, I’ll cover how to use REPL, as well as some interesting quirks of REPL and how to work with them. These workarounds include replacing the underlying mechanism that persists commands, as well as using some command-line editing. And if the built-in REPL doesn’t provide exactly what you need for an interactive environment, there’s also an API to create your own custom REPL.

REPL is an essential Node development tool, and so is the console. We’re using the console in most applications in the book, but there’s more to this helpful object than just logging messages.

REPL: First Looks and Undefined Expressions

To begin REPL, simply type node without providing any Node application file, like so:

$ node

REPL then provides a command-line prompt—an angle bracket (>)—by default. Anything ...