Using a Node repl in Emacs with nvm and npm

Running a repl inside Emacs is often convenient for evaluating code, checking syntax, and myriad other tasks. When I wanted to run a Node REPL, I found that I needed to do a little set up to get everything working the way I wanted.

My first question was: which Node? With nvm, I've installed multiple version on my machine. So I needed a way to specify one to execute.

Another question was: where to run Node? Since npm looks inside node_modules directories starting with the current directory and working up the file system hierarchy, the current working directory is important. If I want access to the npm modules installed for project A, I need to start my repl's Node process from path/to/projectA.

But that raises another question: what happens when I want to switch to project B? Do I need to use process.chdir() to switch the Node repl's current working directory to path/to/projectB? That's clumsy and annoying.

Here's how I answered these questions:

nvm.el gives you nvm-use to activate a version of Node within Emacs. It's basically a nice wrapper around setting the enviroment variables NVM_BIN and NVM_PATH and adding the path to the Node version you want to use to your PATH. Great!

Except for one problem: nvm-use isn't interactive. It's meant to be use programmatically. So I needed to write a small do-nvm-use wrapper that lets me specify a version and then activate it:

(require-package 'nvm)

(defun do-nvm-use (version)
  (interactive "sVersion: ")
  (nvm-use version)
  (exec-path-from-shell-copy-env "PATH"))

To specify where to run Node, I wrote another small defun, named run-node, that prompts for a directory in which to start Node. Before it does this, though, it checks whether a program named node is in the exec-path, and if not, it runs do-nvm-use first. Once we have a Node to execute and a directory to execute it in, we can make a new comint buffer bound to the repl process.

To address the issue of different repls needing to be run for different projects, run-node adds the cwd to the buffer name. Repls for project A and project B will live in buffers named *-node-repl-path/to/projectA and *-node-repl-path/to/projectB, respectively—making switching to the right buffer with ido trivial.

(defun run-node (cwd)
  (interactive "DDirectory: ")
  (unless (executable-find "node")
    (call-interactively 'do-nvm-use))
  (let ((default-directory cwd))
        (pop-to-buffer (make-comint (format "node-repl-%s" cwd) "node" nil "--interactive"))))

Now to start my Node repls, I just call run-node and I'm all set!