So, this is really interesting...

JavaScript is forced to make the pessimal choice of only ever having dynamic-bound "this", because of that favorite whipping boy ... prototypal inheritance.

If JavaScript had a notion of a class (even as a constructor function + prototype) definition, then it would be possible to tag methods with the correct instance reference, as in Ruby and in Python. Unfortunately, because the standard pattern for defining an instance method is:

    Klass.prototype.method = function() {
      ...
    };

... where the function is an expression just like any other, you have to have dynamic-bound this to make prototypes work correctly by default.

You should look at Lua. Yes, if you want a notion of "self" or "this" with prototypal inheritance, then it must be dynamically bound. However, this does not have to be a burden on, or even be noticeable by, the programmer. Good language design can avoid most of the pitfalls in JavaScript...

To elaborate, Lua has special syntax for defining and calling methods vs unbound functions. In Lua,

    function woot:donk() ... end
    woot:donk()

is equivalent to

    woot.donk = function(self) ... end
    woot.donk(woot)

and when you define a function normally e.g.

    function() ... end

there is no self parameter, thus the one from the parent scope is captured. The flaw with JS, really, is that it forces every function to be a method, and doesn't let you express whether you are defining one or the other.

(think I got this right, my Lua is a bit rusty)

Yep, that's pretty much it. And the woot:donk() notation will only evaluate what's left of the colon once, which is nice.

> JavaScript is forced to make the pessimal choice of only ever having dynamic-bound "this", because of that favorite whipping boy ... prototypal inheritance.

I don't think that's true: functions accessed via objects could very well be bound to the object (that's what Python does, accessing a method via an object instance returns a "bound method" which is curried with the instance, whereas accessing the same method via the class returns an "unbound method" which requires a class instance as its first argument).

That works in Python, because Python has a distinction between an "instance" object, and, say, a Dict. In JavaScript an object is an object is an object -- under your proposed change, what would be the value of "this, in this:

    Klass.prototype.method = options.method;

> That works in Python, because Python has a distinction between an "instance" object, and, say, a Dict.

Not really, it's trivial to create a dict-like object which allows access to keys as properties, just override `__getattr__`.

> In JavaScript an object is an object is an object

No, objects can have a non-null [[prototype]] indicating it was created from using `new` on a constructor.

> Klass.prototype.method = options.method;

Depends what `options` is.

This isn't really about prototypal inheritance. If you wrote methods like this in Ruby, you would have the same issue:

    Klass.method := lambda { p self }

Also, if JavaScript looked like this, you wouldn't need the fat arrow:

    // Let's call this a "prototype function"
    Klass.prototype.function method() {
      …
    }

Together with these rules:

* A function always captures its outer `this`

* The getter of a prototype function (`thing.method`) always returns a binded function (where `this` == `thing`).

Absolutely. I don't mean that dynamic "this" is required by prototypal inheritance in general -- I mean that it's required because of JavaScript's particular implementation of prototypes, where they're exposed to you as a value, but there's no syntax where the language can tell that you're in the process of building one out.

You're right - but I'd argue that the difference is just in default behavior, as you touched on in your edit. Ruby procs are bound to their lexical scope, and implicitly include self as a part of that scope. The function still has to be aware of some scope to run, though.

(I didn't know about assigning self in instance_eval, though - that makes me all kinds of happy!)

What I meant to convey is that every language that uses first class functions (I guess I should say "any language with first class functions and the concept of this/self") has to deal with the problem of execution context. Languages like Ruby and Python assume the common case and bundle the context by default. Languages like Javascript and Lua don't, leaving it up to the caller to explicitly specify. In all cases, though, the callee has to be made aware of some context, which is what I meant by the `this` problem. My point is that since Javascript requires explicit binding, a language construct that allows Ruby-style implicit binding is nice syntactic sugar. The author seemed to be conveying the idea that using jQuery's $.proxy magically freed him from the need to pass context around, which is just simply not the case.

What do you mean by Lua leaving the caller to explicitly specify? Are you referring to earlier versions of Lua that used the explicit ^ upvalue sigil? Lua 5.1 (and 5.2) functions close over all local variables (including the implicit “self” introduced by function definitions with colon syntax), with the innermost ones first and no explicit upvalue sigil, much like Scheme.

Nah; regular old Lua 5.1. The colon syntax is just syntactic sugar -- self isn't actually bound.

function Foo:Bar(baz) is the same as Foo.Bar = function(self, baz); invoking Foo:Bar("rebar") is sugar for Foo.Bar(Foo, "rebar"). self is never bound - it's just passed in (explicitly, via . syntax, or implicitly, vs : syntax). In all cases, the caller is always specified.

You can pass Foo.Bar around (as it's a function reference), but if you have something like:

    Foo = {}
    function Foo:Bar(baz)
        print(baz)
    end

    local baz = Foo.Bar

Then baz has no binding information to Foo; defining the function with the : syntax is just syntactic sugar. To invoke, you would have to call:

    baz(Foo, "woohoo")

Just calling

    baz("woohoo")

populates self with "woohoo", and the bar parameter would be nil, demonstrating that there is no contextual binding to the function itself.

What do you mean “isn't actually bound”? The function being defined using colon syntax doesn't close over self, since it's a parameter—but functions defined within that function will close over the self parameter, since it's a local from an enclosing scope:

    foo = { x = 3 }
    function foo:bar(baz)
        return function(thud)
            return thud + baz + self.x
        end
    end

    womble = foo:bar(4)
    womble(7) --> 14

Python I believe also closes over self as a variable, and Ruby has similar behavior for local procs, even though « self » is a special form in Ruby.

This is in distinct contrast to JavaScript, where the value of the special form « this » goes nuts inside closures because it's attached to the function:

    foo = { x: 3 };
    foo.bar = function(baz) {
        return function(thud) {
            return thud + baz + this.x;
        };
    };

    womble = foo.bar(4);
    womble(7) # --> NaN

So in what way are you placing Lua and JavaScript together, and Python and Ruby together?

[slightly rearranged for clarity]

Ah, okay, I see what you mean. Yeah, you're absolutely right there. Point conceded.

Mentally, I was separating Javascript and Lua from Ruby because while the caller is explicitly passed in Javascript and Lua (either via call/apply, or as a parameter), Ruby methods are implicitly aware of their scope (and can't really be referentially passed around like Javascript or Lua methods). Lua "methods" aren't aware of their scope (though closures are.

Ruby methods bound to their objects can be passed around; it's just a bit more cumbersome, in the form of « m = obj.method(:foo) » followed by « m.call(…) ». You can even do « m = :foo.to_proc » in recent Ruby and be able to « m.call(obj, …) »; this is quite useful for things like « [1, 2, 3].inject(&:+) ».

So we have that Python's dot always binds, Ruby's dot always calls, and Lua's dot is always a table lookup, with Lua's colon being a separate syntax for (always) the compound operation with injected self-argument.

And then JavaScript is the schizophrenic one: « obj.foo(bar) » is not the same as « var y = obj.foo; y(bar) ». Property lookup and function call insert a hidden step between them when and only when directly composed, and a function call without an immediately adjacent property lookup in a way injects the opposite step of making « this » in the callee be the global object (I think). “Politicians lie in cast iron sinks.”