Can someone explain this regular array filtering

Question

I am filtering an array and found a regex here. I am trying to figure this out:

filterArray.filter(/./.test.bind(new RegExp(key, 'g'))) 

But I don’t understand how an array checks its value for a regular expression or why you should start with /./ and not just write a regular expression. And how to tie the work in this case?

EDIT: The key is just the string I want to match, “hello” or “dog” or “anything really”.

Answer 1

The .bind() method returns a function with what you passed as the first argument, bound as the value of this .

Since you call .bind() from .test() , you get the .test() method with this binding on new RegExp(key, 'g') .

/./ doesn't matter here. This is just a shortcut to the RegExp.prototype.test method.

As a result, you will really do:

 var regexp = new RegExp(key, 'g'); filterArray.filter(function(val) { return regexp.test(val); }); 

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You should notice that this is a little dangerous, because the regex object with modifier g has state. This means that it always starts a new search where the previous one stopped.

Given this filtering scenario, g doesn't seem to be necessary at all and can really cause problems.

Here is an example of the dangers of using g here:

 var re = /./.test.bind(new RegExp("foo", 'g')); var str = "foobar"; console.log(re(str)); // true console.log(re(str)); // false 

Therefore, calling the same regular expression on the same line leads to two different results. If we call it again, it will be true again.

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Therefore, given the use of .filter() as a callback, let's say key is "foo" , then let's say that one val is "foobar" . This is allowed through the filter.

But let's say that the next val is "foobaz" . The search will resume on the fourth character instead of starting with the first, so "foo" will not be found.

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Here is a concrete example that shows the problem in action:

DEMO: http://jsfiddle.net/s9PzL/

 var filterArray = [ "foobar", "foobaz", "foobuz", "foobix" ]; var result = filterArray.filter(/./.test.bind(new RegExp("foo", 'g'))); 

All lines have "foo" , so they must all go through. But the result shows that this is not happening.

 document.body.textContent = result.join(", "); // foobar, foobuz 

Answer 2

I think the reason is that this is done so because you want to execute a testing method for each element of the array. Just passing the testing method to filter (presumably) will ruin the method binding.

This is why in your example:

 /./ 

Creates an empty regex

 /./.test 

is a test method for this regex

 /./.test.bind(new Regex(..)) 

binds this method to the requested method and returns a new method that executes test , where this is the key based regular expression you provided.

It seems that this could be written much more clearly:

 RegExp.prototype.test.bind(new RegExp(key, 'g'))