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

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.