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

Brendan Eich brendan at
Wed Nov 28 13:15:31 PST 2012


Allen, did this default optional parameter to get() get dropped by accident?


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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