c.js

function C (cb) {
  this.loadConfig(cb)
}

C.prototype.blah = function (arg, cb) {
  if (!this._loaded) {
    this._blah = this._blah || []
    this._blah.push([arg, cb])
    return
  }

  // loaded
  this._blah.forEach(function (x) {
    this.blah(x[0], x[1])
  }, this)
  this._blah = null

  // do actual stuff here.
}