RegExps in array functions

Brendan Eich brendan at
Fri Mar 23 19:12:31 PDT 2012

Felix Böhm wrote:
> Hi guys,
> just a first try for a proposal: Array.prototype.[filter/some/every] 
> currently require a function as an argument. Most of the time, 
> everything I want to do is check if they match a certain pattern. So 
> what I end up writing is a RegExp that matches the pattern, and 
> calling its test() method for every member of the array.
> My proposal is to allow people to use a regular expression as an 
> argument for the three methods. Current implementations throw an error 
> when they don't get a function as an argument, so this won't break any 
> (working) code.
> An example:
> ["fOo","bAr"].filter(function(elem){ return /foo/i.test(elem); });
> should be replaceable with
> ["fOo","bAr"].filter(/foo/i);

Firefox 3 era js shell:

js> ["fOo","bAr"].filter(/foo/i);
js> version()

However, the latest Firefox and js shells do not make RegExp instances 

js> ["fOo","bAr"].filter(/foo/i);
typein:5: TypeError: /foo/i is not a function
js> ["fOo","bAr"].filter(re=/foo/i, re.test.bind(re));
typein:6: TypeError: re is not a function
js> ["fOo","bAr"].filter(function(){return /foo/i});
["fOo", "bAr"]

We removed callable regexps because they violate ES3, as David-Sarah 
Hopwood pointed out 
and related).

> For non-string members of the array, the current behavior of the 
> RegExp.prototype.test method (ie. converting the value to a string) 
> applies.
> Any reasons why this is a bad idea? (Or is it just my bad attitude? :D )

typeof nativeObject == "function" <=> nativeObject is callable

is violated by making regexps callable.


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