Mastering the Node.js REPL, Part One
One of the tools I always had in my belt but never paid too much attention to is the Node.js REPL. REPL stands for Read-Eval-Print-Loop, and that is what it does: It reads an input, evaluates it, prints the result and starts the process again. I only used it when I needed to quickly test a regular expression or try a forgotten API method.
Then I realized I wasn’t taking full advantage of its potential.
The typical way to start the Node.js REPL is by invoking the
node command without arguments. The prompt will change, and you can begin typing. Node expects you to input an expression and will print the result of that expression:
The Node.js REPL also loads all the standard libraries in the global context, so they are available to you:
Another cool trick often forgotten is the autocompletion key. The node REPL autocompletes commands when you hit the
tab key. Unfortunately, this does not work with expressions:
The underscore character
In the Node.js REPL, you can reference the last value using the underscore character
Node.js REPL commands
There are some special commands that you can send to the REPL. These commands start with a dot.
.exit command finishes the REPL session. It’s the same as sending an
End-Of-File by pressing
.load are helpful 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:
Now we can load the session back into the REPL with the
.editor command is particularly helpful when you need to type multi-line content, even though I haven’t yet found a way to navigate up and down the lines:
To get a list of the available commands, you can use the
await within the REPL
await within the REPL is an experimental feature available for Node.js@10 and beyond. You can activate it by passing the special flag
By default, the expression history is persistent. Every expression you evaluate in the REPL is stored in a
.node_repl_history in your
$HOME. This option can be disabled by setting the environment variable
NODE_REPL_HISTORY to an empty string.
Another cool trick you can do with the default REPL is wrapping the
rlwrap. For this, you need to set the
NODE_NO_READLINE to 1. You could make an alias for it:
alias node="env NODE_NO_READLINE=1 rlwrap node
Now, if you hit ctrl-r, you can search your command history.
Wrapping it up
In this post, we have covered the basics of the default node REPL. In the second part of this series, we will use the standard Node.js
repl library to customize it.