How can a set of CSS selectors be sorted on the basis of CSS specificity in a JS function?
function SortByCssSpecificity(input_array_of_css_s开发者_如何学编程electors) {
...
return sorted_array_of_css_selectors;
}
From the Selectors level 3 spec:
A selector's specificity is calculated as follows:
- count the number of ID selectors in the selector (= a)
- count the number of class selectors, attributes selectors, and pseudo-classes in the selector (= b)
- count the number of type selectors and pseudo-elements in the selector (= c)
- ignore the universal selector
Selectors inside the negation pseudo-class [
:not()
] are counted like any other, but the negation itself does not count as a pseudo-class.Concatenating the three numbers a-b-c (in a number system with a large base) gives the specificity.
Examples:
* /* a=0 b=0 c=0 -> specificity = 0 */ LI /* a=0 b=0 c=1 -> specificity = 1 */ UL LI /* a=0 b=0 c=2 -> specificity = 2 */ UL OL+LI /* a=0 b=0 c=3 -> specificity = 3 */ H1 + *[REL=up] /* a=0 b=1 c=1 -> specificity = 11 */ UL OL LI.red /* a=0 b=1 c=3 -> specificity = 13 */ LI.red.level /* a=0 b=2 c=1 -> specificity = 21 */ #x34y /* a=1 b=0 c=0 -> specificity = 100 */ #s12:not(FOO) /* a=1 b=0 c=1 -> specificity = 101 */
(Selectors level 4, published after this answer, adds another layer of complexity to specificity thanks to the introduction of some new selectors that is currently outside this answer's scope.)
Here's a pseudocode implementation to get you started, it is nowhere near perfect but I hope it's a reasonable starting point:
function SortByCssSpecificity(selectors, element) {
simple_selectors = [][]
for selector in selectors {
// Optionally pass an element to only include selectors that match
// The implementation of MatchSelector() is outside the scope
// of this answer, but client-side JS can use Element#matches()
// https://developer.mozilla.org/en-US/docs/Web/API/Element/matches
if (element && !MatchSelector(selector, element)) {
continue
}
simple_selectors[selector] = ParseSelector(selector)
simple_selectors[selector] = simple_selectors[selector].filter(x | x != '*')
// This assumes pseudo-elements are denoted with double colons per CSS3
// A conforming implementation must interpret
// :first-line, :first-letter, :before and :after as pseudo-elements
a = simple_selectors[selector].filter(x | x ^= '#').length
b = simple_selectors[selector].filter(x | x ^= '.' or x.match(/^:[^:]+/) or x.match(/^\[.+\]$/)).length
c = simple_selectors[selector].length - (a + b)
simple_selectors[selector][count] = parseInt('' + a + b + c)
}
return simple_selectors.sort(x, y | x[count] < y[count])
}
function ParseSelector(selector) {
simple_selectors = []
// Split by the group operator ','
// Split each selector group by combinators ' ', '+', '~', '>'
// :not() is a special case, do not include it as a pseudo-class
// For the selector div > p:not(.foo) ~ span.bar,
// sample output is ['div', 'p', '.foo', 'span', '.bar']
return simple_selectors
}
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