Brendan Eich brendan at
Fri Nov 2 16:57:14 PDT 2012

Andrea Giammarchi wrote:
> I actually agree on this, but Set.has() is misleading then, imho. 
> Shouldn't be specified that `has()` is key related and `contains()` is 
> value related?

The forEach signature has (value, key, collection), and for a Set is 
called with (value, value, theSet). We discussed this at the Boston f2f 
and recently on list. It allows us to extend Sets in the future (under 
object model reformation, e.g.) to map value to boolean has-result.

Apart from the Set special case, using contains for values and has for 
keys seems better and better to me. But that means 
String.prototype.contains, not has. The test is for values, not keys, in 
that case. Same thing with Array.prototype.contains (and MooTools must 


> Also a "magic" Object.has(obj, key) with smart obj recognition and 
> Object.contains(obj, value) would be probably cool
> On Fri, Nov 2, 2012 at 2:30 PM, Allen Wirfs-Brock 
> <allen at <mailto:allen at>> wrote:
>     On Nov 2, 2012, at 10:56 AM, Jason Orendorff wrote:
>>     On Fri, Nov 2, 2012 at 12:29 PM, Domenic Denicola
>>     <domenic at
>>     <mailto:domenic at>> wrote:
>>         > If we call it "has", should we also rename
>>         String.prototype.contains?
>>         I'd say no; the distinction between collections "having" an
>>         element and strings "containing" a substring seems very
>>         sensible. It's a bit more awkward to say a string "has" a
>>         substring, and a string is definitely not a collection of
>>         substrings in any reasonable sense.
>>     Well, you could also note that array.has(x) looks for a
>>     particular value, while map.has(x) looks for a particular key.
>>     But that's not the point. There's no common formal contract
>>     implemented by all these methods; what they share is an informal
>>     "hey, look in this thing, and tell me if you see that thing" vibe.
>     There is no verifiable formal contract.  But there can be an
>     informal contract.  In my experience, it is very important when
>     using a dynamic language to recognize and try to support such
>     informal contracts.
>     All the uses of "has" we have defined so far are about the "keys".
>      Having one place that is about the "values" to create unnecessary
>     confusion
>     Note that we (the JS/ES designers) already have a history of being
>     being inconsistent in our use of names.  Consider String indexOf
>     and Array indexOf they are named the same and appear to have
>     signatures.  But they logically are doing quite different things.
>      String indexOf is looking for the index of the first element of a
>     subsequence of character elements that matches a specific
>     character sequence.  Array indexOf is looking for the index of a
>     single element that contains a specific value.  You might want to
>     implement a logically similar subsequence search for Array's but
>     if you do, you can't call it indexOf because that name was already
>     used for something with different semantics.  We should try to do
>     better as people for our example.
>     Allen
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