Angus Croll wrote:
On Sat, Apr 21, 2012 at 3:06 PM, Axel Rauschmayer <[email protected]
<mailto:[email protected]>> wrote:
On Sat, Apr 21, 2012 at 1:56 PM, Axel Rauschmayer
<[email protected] <mailto:[email protected]>> wrote:
Counter-question: Isn’t it clear when you create a function
whether it is going to be a non-method function or a method?
It's clear to the implementer - Its not clear to a function that
gets it as an argument - unless fn.prototype is checked
I would argue that – in ES.next – whenever a function is passed as
an argument, one should always assume that it is a non-method
function and completely ignore `this`.
This makes me sad. Seems like functions are going to be a little less
first class than they were before
There is no "before". Today, no ES6, you pass to another module written
by someone else (or by an older and forgotten "you") a function that (a)
you wrote and (b) uses |this|, you had better .bind(that) or otherwise
make sure it receives the right |this|.
People fail to do this all the time. The easiest bug is just to use
|this| in a function expression assuming it will be the same as in the
enclosing function. Seasoned programmers make this mistake, it's the
safety-last nature of always-dynamic |this|.
Given the requirement *today* that you bind |this| or don't use it,
there's nothing less "first class" in the future. The problem exists
today, and indeed some analyses showed 80-90% or more of functions
either bind |this| or do not use it. These are all candidates for arrow.
But if we make arrows allow |this| override, then the roughly half of
that 80-90% that want to bind |this| cannot be sure a caller won't
override via .apply or .call (why not through a method reference too?).
We'll be back in the "I forgot to use .bind(that) or var that = this; on
the outside, the footgun fired all too often.
- and for fairly arbitrary reasons (the introduction of tight-bound
|this| in one syntax varietal).
There's nothing arbitrary about fixing the hazard with new syntax that
reliably binds |this|. The argument to have is whether -> should be
added too.
And not being able to assume call and apply will work on any given
function feels like the road to deprecation.
You cannot assume |this| can be overridden by apply and call today:
js> function f(){"use strict"; return this.x}
js> f.apply({x:42})
42
js> g = f.bind({x:99})
function f() {[native code]}
js> g()
99
js> g.apply(101)
99
(No ES6 here, and ES5 not required -- bind preceded ES5 by many years
and can be written in JS.)
Assuming something not guaranteed is a mistake. The fix is not to keep
assuming and propagate the assumption (that |this| can be overridden) to
new and better short-function forms.
/be
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