fab.arity.js

// async: manually sets the length of a function by wrapping it in another with the desired length

fab.arity = function( num ) {
  var fn = function(_){ fn.apply( this, arguments ) }
    , ends = [ "this(", ")" ]
    , args = [];

  while ( --num ) args[ num - 1 ] = "_" + num;

  this( Function( "fn", ends.join( fn ).replace( "_", args ) ) );
}