Execute code each time the Node.js REPL starts

If you start the Node.js binary without any arguments, you are in the REPL (Read-Eval-Print-Loop), a JavaScript command line. This post shows you how to execute code each time the REPL starts. That allows you to, say, automatically load modules you want to use.

I have not found a way to perform the code execution via a configuration file, but you can make it happen via the following trick: Write a script that executes the code and then programmatically starts a REPL. Example: The following are the contents of a file called startnode.js.

    #!/usr/local/bin/node

    var repl = require("repl");
    var context = repl.start("$ ").context;
    
    // Configure what’s available in the REPL
    context.util = require("util");

Under Unix, the first line lets you run startnode.js as a command, if you make it executable.

    rauschma> chmod u+x startnode.js 
    rauschma> ./startnode.js
    $ util.log("hello")
    17 Nov 21:39:29 - hello

You can also use the following command to run it (with or without the first line):

    rauschma> node startnode.js

The Node.js documentation has more information on the REPL API and on commands that are available while running it. For example:

    $ .help
    .break	Sometimes you get stuck, this gets you out
    .clear	Break, and also clear the local context
    .exit	Exit the repl
    .help	Show repl options

Related post:

• Tip: load source from a file in the Node.js shell