Default value argument for Map/WeakMap.prototype.get()

Allen Wirfs-Brock allen at
Wed Nov 28 13:46:50 PST 2012

It isn't in the wiki proposal, so it probably didn't get included when I created those  chapter 15 sections.  If there is consensus on those optional arguments I can add them.


On Nov 28, 2012, at 1:15 PM, Brendan Eich wrote:

> Agreed.
> Allen, did this default optional parameter to get() get dropped by accident?
> /be
> Tab Atkins Jr. wrote:
>> On Wed, Nov 28, 2012 at 12:12 PM, Andrea Giammarchi
>> <andrea.giammarchi at>  wrote:
>>> apologies, I meant it is trivial, but it depends what you are looking for /
>>> need
>>> So, since is that easy to create your own case, then no reason to add the
>>> default argument in the API
>> No, that's not the case.  Even if it's "trivial" to write it on your
>> own, if its use-case is sufficiently common, adding it helps with the
>> overall usability of the language.  After all, by your argument, we
>> don't need Map#has either, since you can get around it by just doing:
>> function mapset(map, key, val) { map.set(key, [val]); }
>> function mapget(map, key) { return map.get(key)[0]; }
>> function maphas(map, key) { return map.get(key) === undefined; }
>> This lets you store undefined in a map, and still distinguish it from
>> the key not being set at all, while using solely Map#get and Map#set,
>> and a trivial bit of code.
>> In this case, I agree that getting a default value from a map when the
>> key is undefined is extremely common (I do it regularly when using
>> dictionaries in Python), and it should be part of the default API.
>> ~TJ
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