arity based dispatch

dispatch.js

function dispatcher(methods, fallback) {
  function dispatch() {
    var method = dispatch.methods[arguments.length] || fallback
    if (typeof(method) !== 'function')
      throw Error('Arity is not supported')
    return method.apply(this, arguments)
  }
  dispatch.methods = methods.reduce(function(methods, method) {
    methods[method.length] = method
    return methods
  }, [])
  return dispatch
}

usage.js

var sum = dispatcher([
  function() { return 0 },
  function(x) { return x },
  function(x, y) { return x + y }
], function(x, y, z) {
  return Array.prototype.slice.call(arguments, 1).reduce(function(x, y) {
    return sum(x, y)
  }, x)
})

sum()         // => 0
sum(2)        // => 2
sum(2, 3)     // => 5
sum(2, 3, 4)  // => 9

It would be intuitive to take advantage of ES.next ...rest and write code like this:

var sum2= dispatcher([
  function() { return 0 },
  function(x) { return x },
  function(x, y) { return x + y },
  function(x, ...rest) { return rest.reduce(function(x, y) { return x + y }) }
])

sum2(2, 3, 4)     // =>  Error: Exception: Arity is not supported

Which won't do what's user expects, since length of last element function will be 1.

No, the .length of the last element function is 1.

@dherman I know, that actually why I wrote this, I was trying to illustrate why we need an API for testing if ...rest is used. Here is related es discussion: https://mail.mozilla.org/pipermail/es-discuss/2012-June/023277.html

Oh I see I actually wrote in my comment that function length will be 2 instead of 1. I'm updating it now.