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Zest - a fast, lightweight, and extensible CSS selector engine

Zest is a CSS selector engine. I originally wrote it in a night as a proof of concept for certain non-standard selectors I was interested in, but once I noticed how fast it was before I had even optimized it, I decided to develop it a bit more.

Zest was designed to be very concise while still supporting CSS3/CSS4 selectors and remaining fast. For context, when compared to the well-known Sizzle engine:

  • Zest - 400-500 lines, and ~200 semicolons.
  • Sizzle - 1413 lines and 413 semicolons.


Each selector run 1000 times on Google Chrome 13 beta (ms):

benchmarking: `header > h1` 1000 times.
zest: 13
sizzle: 24
native: 13
benchmarking: `body > header > h1` 1000 times.
zest: 16
sizzle: 26
native: 13
benchmarking: `html a` 1000 times.
zest: 45
sizzle: 55
native: 12
benchmarking: `:first-child` 1000 times.
zest: 44
sizzle: 68
native: 11
benchmarking: `:only-child` 1000 times.
zest: 49
sizzle: 66
native: 12
benchmarking: `:not(a)` 1000 times.
zest: 51
sizzle: 125
native: 12
benchmarking: `h1 + time:last-child` 1000 times.
zest: 15
sizzle: 32
native: 13
benchmarking: `h1 + time[datetime]:last-child` 1000 times.
zest: 21
sizzle: 45
native: 14
benchmarking: `header > h1, :not(a)` 1000 times.
zest: 72
sizzle: 212
native: 17
benchmarking: `a[rel~="section"]` 1000 times.
zest: 41
sizzle: 54
native: 11
benchmarking: `a, h1` 1000 times.
zest: 25
sizzle: 55
native: 11
benchmarking: `:nth-child(2n+1)` 1000 times.
zest: 82
sizzle: 97
native: 13

NOTE: If you want to run these benchmarks yourself make sure to turn off Sizzle's (and Zest's) document.querySelectorAll delegation mechanism, otherwise you will be benchmarking against document.querySelectorAll.

Zest will cache compiled selectors if it can't delegate to document.querySelectorAll, document.getElementById, or document.getElementsByClassName (depending). The benchmark tests you see above were performed with the caching mechanism disabled. If caching were enabled, Zest would be faster than the native document.querySelectorAll.


// second parameter is the context
zest('header > h1', document); 


Zest currently includes support for ender.js, Prototype, and jQuery.

Unsupported Selectors: :nth-last-of-type, :nth-last-child, :nth-last-match, :hover, :link, :visited, all pseudo elements, and namespaces.

:link, :visited, and pseudo elements are unsupported for obvious reasons (they don't work). :hover isn't supported because it examines a dynamic state, you should be binding to events for this (:focus is supported, but there is no fallback for legacy browsers). The :nth-last-x pseudo-classes are unsupported simply because I haven't gotten around to adding them, and because they add a lot of seemingly unnecessary code. I don't plan on adding namespace selector support currently, but it may be added eventually.

Zest tries to include support for selectors level 4, but the spec is really up in the air right now. Currently, :scope, :links-here, :matches, and :nth-match are all supported. Support for the subject selector prefix will be added when the spec becomes more stable.

This selector engine is still relatively new, if there are any bugs please submit them to the issue tracker.


Zest doesn't support any non-standard selectors, but it is possible to add your own.

Adding a simple selector

Adding simple selectors is fairly straight forward. Only the addition of pseudo classes and attribute operators is possible. (Adding your own "style" of selector would require changes to the core logic.)

Here is an example of a custom :name selector which will match for an element's name attribute: e.g. h1:name(foo). Effectively an alias for h1[name=foo].

// if there was a parameter, 
// it gets closured as `param`
zest.selectors[':name'] = function(param) {
  return function(el) {
    if ( === param) return true;

NOTE: if you're pseudo-class does not take a parameter, there will be no closure.

Adding an attribute operator

// `attr` is the attribute
// `val` is the value to match
zest.operators['!='] = function(attr, val) {
  return attr !== val;

Adding a combinator

Adding a combinator is a bit trickier. It may seem confusing at first because the logic is upside-down. Zest interprets selectors from right to left.

Here is an example how a parent combinator could be implemented:

zest.combinators['<'] = function(test) {
  return function(el) { // `el` is the current element
    el = el.firstChild;
    while (el) {
      // return the relevant element
      // if it passed the test
      if (el.nodeType === 1 && test(el)) {
        return el;
      el = el.nextSibling;

The test function tests whatever simple selectors it needs to look for, but it isn't important what it does. The most important part is that you return the relevant element once it's found.

NOTE: Using alphanumeric characters as combinators will break the descendant combinator.

Don't do this: header X h1. With X being the combinator. It creates ambiguity as to whether X is an qname or not.


(c) Copyright 2011, Christopher Jeffrey (MIT Licensed).
See LICENSE for more info.