Append extra arguments to a function

library.js

function a(a1, callback) {
  // ...does stuff

  callback("a", "b", "c")
}

stuff.js

// Returns a function that, when called, calls `fun` with first the arguments passed
// to the returned function, and then any additional bound arguments appended *after*
// that...  Sort-of like a reverse `Function.prototypee.bind`.
function appendArgsTo(fun/*, ...extraArgs*/) {
  var extraArgs = Array.prototype.slice.call(arguments, 1)

  return function(/*...args*/) {
    var args = Array.prototype.slice.call(arguments)
    
    return fun.apply(this, args.concat(extraArgs))
  }
}

// ...and then invoke it
a("something", appendArgsTo(callback, "D!!!"))