rpl for the future and past. An alternative to the node default REPL (what you access when you just call node and can type in lines of code).

npm install -g rpl

The main trick is that this supports time travel. You can instrument code calls by using special comments, and edit previous code, changing future values.

It also supports async instrumentation, since node is node.

require('fs').readFile('/etc/hosts', 'utf8', function(err, res) {
  //=res
});

rpl [-b] [-o] [FILENAME]
      ^   ^    ^
      |   |    |
      |   |    | prefill the cli with the contents of a file
      |   |
      |   | -o open your browser to the page
      |
      | -b open a chrome app standalone window

rpl is a node module you install globally. When you run rpl, it starts up a server at http://localhost:3000/, so you'll need to open your browser to that page. From there you just type, and when you want to inspect a value, add a comment to your source code like:

//=variableName

You can also type expressions there, like:

//=Date()

You can use require() just like you would elsewhere, and, like the node REPL, require() calls become relative to the current working directory.

rpl is the sibling of mistakes.io, something that does something similar but in browsers instead of node and implicitly instead of explicitly.