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a CSS selector compiler/engine


css-select turns CSS selectors into functions that tests if elements match them. When searching for elements, testing is executed “from the top”, similar to how browsers execute CSS selectors.

In its default configuration, css-select queries the DOM structure of the domhandler module (also known as htmlparser2 DOM).



The traditional approach of executing CSS selectors, named left-to-right execution, is to execute every component of the selector in order, from left to right (duh). The execution of the selector a b for example will first query for a elements, then search these for b elements. (That’s the approach of eg. Sizzle, nwmatcher and qwery.)

While this works, it has some downsides: Children of as will be checked multiple times; first, to check if they are also as, then, for every superior a once, if they are bs. Using Big O notation, that would be O(n^(k+1)), where k is the number of descendant selectors (that’s the space in the example above).

The far more efficient approach is to first look for b elements, then check if they have superior a elements: Using big O notation again, that would be O(n). That’s called right-to-left execution.

And that’s what css-select does – and why it’s quite performant.

How does it work?

By building a stack of functions.

Wait, what?

Okay, so let’s suppose we want to compile the selector a b again, for right-to-left execution. We start by parsing the selector, which means we turn the selector into an array of the building-blocks of the selector, so we can distinguish them easily. That’s what the css-what module is for, if you want to have a look.

Anyway, after parsing, we end up with an array like this one:

  { type: 'tag', name: 'a' },
  { type: 'descendant' },
  { type: 'tag', name: 'b' }

Actually, this array is wrapped in another array, but that’s another story (involving commas in selectors).

Now that we know the meaning of every part of the selector, we can compile it. That’s where it becomes interesting.

The basic idea is to turn every part of the selector into a function, which takes an element as its only argument. The function checks whether a passed element matches its part of the selector: If it does, the element is passed to the next turned-into-a-function part of the selector, which does the same. If an element is accepted by all parts of the selector, it matches the selector and double rainbow ALL THE WAY.

As said before, we want to do right-to-left execution with all the big O improvements nonsense, so elements are passed from the rightmost part of the selector (b in our example) to the leftmost (which would be c of course a).

//TODO: More in-depth description. Implementation details. Build a spaceship.


var CSSselect = require("css-select");

CSSselect(query, elems, options)

Queries elems, returns an array containing all matches.

Aliases: CSSselect.selectAll(query, elems), CSSselect.iterate(query, elems).


Compiles the query, returns a function.

CSSselect.is(elem, query, options)

Tests whether or not an element is matched by query. query can be either a CSS selector or a function.

CSSselect.selectOne(query, elems, options)

Arguments are the same as for CSSselect(query, elems). Only returns the first match, or null if there was no match.


Supported selectors

As defined by CSS 4 and / or jQuery.

*: Not part of CSS3

License: BSD-like