http://wiki.ecmascript.org/doku.php?id=strawman:block_lambda_revival

improves-scope-binding.js

// JS Today
function List() {
  this.list = [].slice.call(arguments);
}


List.prototype.diff = function( array ) {
  return array.filter(function(item) {
    return !~this.list.indexOf(item);
  }, this);
};


var a = new List(1, 2, 3, 4, 5, 6);

a.diff([ 0, 2, 4, 6, 8, 10 ]);
// [ 0, 8, 10 ]

// Block Lambda and Rest Params
function List( ...args ) {
  this.list = args;
}

List.prototype.diff = function( array ) {
  return array.filter({ |item|
    !~this.list.indexOf(item);
  });
};

@ryan that only flies if paren-free makes the cut. I meant to be conservative about what is being illustrated