BTF Measurements

John J Barton johnjbarton at
Tue Mar 13 09:38:17 PDT 2012

On Tue, Mar 13, 2012 at 9:25 AM, Kevin Smith <khs4473 at> wrote:
>> I'm confused by this paragraph. Functions that benefit from 'this'
>> binding would reference 'this' or an alias for 'this' right?
> In this analysis, functions that reference an "alias" to this (e.g. var self
> = this) within an inner scope are counted:
>     (function() {
>         var self = this;
>         (function() { console.log(self); });
>     });
> This results in 1 BTF candidate (the inner function).

Then does it count this:
     (function() {
         var self = this;
         (function() { console.log("foo"); });

Can you answer the 'cost' side of the BTF issue? That is if BTF was
really easy, how many times would we have to opt-out so we can use the
default value of "this"? My guess: you will need a lot zeros in place
of X  in 0.0X% ;-)

Sometimes the 'this' is not needed at all; does it ever matter that it
is set to the lexical this?

> In my analysis, I mention that this methodology will tend to undercount the
> BTF candidates, because functions with explicitly bound "this" (through
> Function.prototype.bind or other means) will tend to get marked as
> "methods":
>     element.onclick = function() {
>         console.log(this);
>     }.bind(this);

Well FWIW I stop using "var self" the day bind() landed.


> The analyzer reports 0 BTF candidates, even though we clearly should have 1.
>  Since the tool undercounts, we can say that the reported numbers offer a
> lower bound on BTF candidacy.  The fact that BFT candidacy may be higher
> doesn't alter the analysis.
> kevin

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