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);
  });
};

So in the block lambda revival this has the same meaning as the outer function?

Yes, the scope inside the block lambda is set to the scope of the LexicalEnvironment where it is running - which is awesome for avoiding var that = this; and .bind(this);

Also, +9001 for being identifiable (the pipes look weird at first, but they've grown on me in their ability to clearly illustrate what this syntax is doing) and for being way less keystrokes to get the job done

I don't like the implicit return, but besides that it's neat.

Well, maybe this should be updated to include an example, but you can use an explicit return as well.

Why the () around it? Why not

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

Think about this

List.prototype.diff = function( array ) {
  return array.filter { |item|
    !~this.list.indexOf(item);
  }.sort { |a, b| 
    // Do something with a and b
  };
};

I'm not a fan of }.

Rick,

Note that you don't need the bind() call in ES5. The 2nd argument to filter() becomes the this value in the predicate.
I still want blocks, though!

Why the () around it?

In that case JS either gets implicit parens for function calls or block lambda becomes a special case you have to explain to every new js developer.

@david, ya know... I knew that and completely forgot thank you for pointing it out. I will update asap.

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