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A CSS selector compiler and engine


As a compiler, css-select turns CSS selectors into functions that tests if elements match them.

As an engine, css-select looks through a DOM tree, searching for elements. Elements are tested “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). To query alternative DOM structures, see Options below.



Most CSS engines written in JavaScript execute selectors left-to-right. That means they execute every component of the selector in order, from left to right. As an example: For the selector a b, these engines will first query for a elements, then search these for b elements. (That’s the approach of eg. Sizzle, Qwery and NWMatcher.)

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, for right-to-left execution. We start by parsing the selector. This turns the selector into an array of the building blocks. 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 is where things become 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 function representing the next part of the selector. That function 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. That means elements are passed from the rightmost part of the selector (b in our example) to the leftmost (which would be c of course a).

For traversals, such as the descendant operating the space between a and b, we walk up the DOM tree, starting from the element passed as argument.

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


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

Note: css-select throws errors when invalid selectors are passed to it. This is done to aid with writing css selectors, but can be unexpected when processing arbitrary strings.

CSSselect.selectAll(query, elems, options)

Queries elems, returns an array containing all matches.

Aliases: default export, CSSselect.iterate(query, elems).

CSSselect.compile(query, options)

Compiles the query, returns a function., 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.selectAll(query, elems). Only returns the first match, or null if there was no match.


All options are optional.

Custom Adapters

A custom adapter must match the interface described here.

You may want to have a look at domutils to see the default implementation, or at css-select-browser-adapter for an implementation backed by the DOM.

Supported selectors

As defined by CSS 4 and / or jQuery.

License: BSD-2-Clause

Security contact information

To report a security vulnerability, please use the Tidelift security contact. Tidelift will coordinate the fix and disclosure.

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