css-select

A CSS selector compiler and engine

What?

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.

Features:

• 🔬 Full implementation of CSS3 selectors, as well as most CSS4 selectors
• 🧪 Partial implementation of jQuery/Sizzle extensions (see
  cheerio-select for the
  remaining selectors)
• 🧑‍🔬 High test coverage, including the full test suites from
  Sizzle,
  Qwery and
  NWMatcher and .
• 🥼 Reliably great performance

Why?

Most CSS engines written in JavaScript execute selectors left-to-right. That
means thet 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.

API

 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.

• query can be either a CSS selector or a function.
• elems can be either an array of elements, or a single element. If it is an
  element, its children will be queried.
• options is described below.

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

CSSselect.compile(query, options)

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

Options

All options are optional.

• xmlMode: When enabled, tag names will be case-sensitive. Default: false.
• rootFunc: The last function in the stack, will be called with the last
  element that's looked at.
• adapter: The adapter to use when interacting with the backing DOM
  structure. By default it uses the domutils module.
• context: The context of the current query. Used to limit the scope of
  searches. Can be matched directly using the :scope pseudo-class.
• relativeSelector: By default, selectors are relative to the context,
  which means that no parent elements of the context will be matched. (Eg.
  a b c with context b will never give any results.) If relativeSelector
  is set to false, selectors won't be
  absolutized and selectors
  can test for parent elements outside of the context.
• cacheResults: Allow css-select to cache results for some selectors,
  sometimes greatly improving querying performance. Disable this if your
  document can change in between queries with the same compiled selector.
  Default: true.
• pseudos: A map of pseudo-class names to functions or strings.

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.

• Selector lists
  (,)
• Universal
  (*)
• Type
  (<tagname>)
• Descendant
  ( )
• Child
  (>)
• Parent (<)
• Adjacent sibling
  (+)
• General sibling
  (~)
• Attribute
  ([attr=foo]), with supported comparisons:
  • [attr] (existential)
  • =
  • ~=
  • |=
  • *=
  • ^=
  • $=
  • !=
  • i and s can be added after the comparison to make the comparison
    case-insensitive or case-sensitive (eg. [attr=foo i]). If neither is
    supplied, css-select will follow the HTML spec's
    case-sensitivity rules.
• Pseudos:
  • :not
  • :contains
  • :icontains (case-insensitive version of :contains)
  • :has
  • :root
  • :empty
  • :parent
  • :first-child,
    :last-child,
    :first-of-type,
    :last-of-type
  • :only-of-type,
    :only-child
  • :nth-child,
    :nth-last-child,
    :nth-of-type,
    :nth-last-of-type,
  • :link,
    :any-link
  • :visited,
    :hover,
    :active
    (these depend on optional Adapter methods, so these will only match
    elements if implemented in Adapter)
  • :selected,
    :checked
  • :enabled,
    :disabled
  • :required,
    :optional
  • :header,
    :button,
    :input,
    :text,
    :checkbox,
    :file,
    :password,
    :reset,
    :radio etc.
  • :is, plus its
    legacy alias :matches
  • :scope
    (uses the context from the passed options)

---

License: BSD-2-Clause

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