jQuery has a lovely if somewhat misnamed method called closest() that walks up the DOM tree looking for a matching element. For example, if I've got this HTML:
<table src="foo">
<tr>
<td>Yay</td>
</tr>
</table>
Assuming element
is set to <td>
, then I can figure the value of src
like this:
element.closest('table')['src']
And that will cleanly return "undefined" if either of the table element or its src attribute are missing.
Having gotten used to this in Javascriptland, I'd love to find something equivalent for Nokogiri in Rubyland, but the closest I've been able to come up with is this distinctly inelegant hack using ancestors():
ancestors = element.ancestors('table')
src = ancestors.any? ? first['src'] : nil
The ternary is needed because first returns nil if called on an empty array. Better ideas?
jQuery closest() Method The closest() method returns the first ancestor of the selected element. An ancestor is a parent, grandparent, great-grandparent, and so on.
The closest() is an inbuilt method in jQuery that returns the first ancestor of the selected element in the DOM tree. This method traverse upwards from the current element in the search of first ancestor of the element.
So the opposite of closest(), searching down, would be find().
The closest() method searches up the DOM tree for elements which matches a specified CSS selector. The closest() method starts at the element itself, then the anchestors (parent, grandparent, ...) until a match is found. The closest() method returns null() if no match is found.
You can call first
on an empty array, the problem is that it will return nil
and you can't say nil['src']
without getting sad. You could do this:
src = (element.ancestors('table').first || { })['src']
And if you're in Rails, you could use try
thusly:
src = element.ancestors('table').first.try(:fetch, 'src')
If you're doing this sort of thing a lot then hide the ugliness in a method:
def closest_attr_from(e, selector, attr)
a = e.closest(selector)
a ? a[attr] : nil
end
and then
src = closest_attr_from(element, 'table', 'src')
You could also patch it right into Nokogiri::XML::Node (but I wouldn't recommend it):
class Nokogiri::XML::Node
def closest(selector)
ancestors(selector).first
end
def closest_attr(selector, attr)
a = closest(selector)
a ? a[attr] : nil
end
end
You can also do this with xpath:
element.xpath('./ancestor::table[1]')
You want the src
attribute of the closest table ancestor, if it exists? Instead of getting an element that might exist via XPath and then maybe getting the attribute via Ruby, ask for the attribute directly in XPath:
./ancestor::table[1]/@src
You'll get either the attribute or nil:
irb(main):001:0> require 'nokogiri'
#=> true
irb(main):002:0> xml = '<r><a/><table src="foo"><tr><td /></tr></table></r>'
#=> "<r><a/><table src=\"foo\"><tr><td /></tr></table></r>"
irb(main):003:0> doc = Nokogiri.XML(xml)
#=> #<Nokogiri::XML::Document:0x195f66c name="document" children=…
irb(main):004:0> doc.at('td').at_xpath( './ancestor::table[1]/@src' )
#=> #<Nokogiri::XML::Attr:0x195f1bc name="src" value="foo">
irb(main):005:0> doc.at('a').at_xpath( './ancestor::table[1]/@src' )
#=> nil
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