with_file.coffee

fs = require 'fs'

class MyClass

  # Trying to create middleware with access to class members.
  withData: (cb) -> 
    return cb @__data if @__data?
    fs.readFile 'datafile.json', (err, json) =>
      throw err if err?
      @__data = JSON.parse json
      cb @__data

  getUsers: (n) -> @withData (data) =>
    data.users[..n-1]

x = new MyClass
console.log x.getUsers 5

Also, this is no longer bound to the instance within getUsers when defined via the class method.

Why so much magic? Why not just call withData from inside the function?

In my actual use case, I have many functions that rely on async data and I just wanted to avoid the extra level of indentation.

@tdreyno, I found a less magical version that works – it's just what you recommended except I call the 'middleware' right after the argument list.