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A Usable Node.js REPL for Emacs

Posted by Jakub Holý on March 11, 2015

Being used to the excellent REPL in Clojure(Script), I was surprised to find out that Node.js REPL is somewhat weak and that its support in Emacs is not actively maintained. I anyway managed to get a usable REPL with these three components:

  1. The Emacs nodejs-repl package (nearly 2 years old)
  2. J. David Smith’s nodejs-repl-eval.el to be able to send code to the REPL (binding nodejs-repl-eval-dwim to C-x C-e so that I can execute the current sexp/region)
  3. My own extension of nodejs-repl-eval.el that takes care of escaping JS constructs that the REPL interprets in a special way

Regarding #3: The problem with the Node.js REPL is that valid JS code does not always behave correctly in the REPL. This is because: 1) _ is a special variable (the last result) while in code it is often used for the underscore/lodash library; 2) The REPL also interprets lines somewhat separately and tries to execute <dot><name> as a REPL command, breaking chained calls that start on a new line. My solution uses some RegExp magic to turn

var _ = require("lodash"); // #1a conflicting use of _
_.chain([1,2])             // #1b conflicting use of _
   .first()                // #2 interpreted as non-existing REPL command '.first'


var __ = require("lodash");  // #1a Notice the doubled _
__.chain([1,2]).             // #1b Notice the doubled _
   first().                  // #2 Notice the dot has moved to the previous line

when the code is being sent to the REPL

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