From https://plus.google.com/+IanBicking/posts/PbXDtNF9Gg6:

"""
Ian Bicking
Shared publicly - Mar 24, 2015
#Array

Huh, "for (attr in obj)" goes along with "if (attr in obj)", but "for 
(item of array)" doesn't have an equivalent "if (item of array)"

There is a proposal I guess for a method, but only for ES7: 
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes
"""

It's obvious in hindsight. Wants a @@hasInstance-like protocol, not just 
desugaring to `includes` with operands transposed to receiver and argument.

An `of` operator with unstratified symbol-named MOP hook helps various 
container-like objects cope with the vagaries of NaNs and indexOf / 
includes differences between arrays and strings, and (most winningly) 
lines up with for-of, as Ian points out, in a parallel to for-in and `in`.

I'm sure it's worth some debate, perhaps someone will k.o. the idea with 
an objection I can't think of, but I like it.

/be

From https://plus.google.com/+IanBicking/posts/PbXDtNF9Gg6:

> Ian Bicking  
> Shared publicly - Mar 24, 2015

> #Array

> Huh, "for (attr in obj)" goes along with "if (attr in obj)", but "for 
> (item of array)" doesn't have an equivalent "if (item of array)"

> There is a proposal I guess for a method, but only for ES7: 
> https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/includes

It's obvious in hindsight. Wants a `@@hasInstance`-like protocol, not just 
desugaring to `includes` with operands transposed to receiver and argument.

An `of` operator with unstratified symbol-named MOP hook helps various 
container-like objects cope with the vagaries of NaNs and indexOf / 
includes differences between arrays and strings, and (most winningly) 
lines up with for-of, as Ian points out, in a parallel to for-in and `in`.

I'm sure it's worth some debate, perhaps someone will k.o. the idea with 
an objection I can't think of, but I like it.