index.js

/**
 * Originally ported from https://github.com/keeganstreet/specificity/blob/866bf7ab4e7f62a7179c15b13a95af4e1c7b1afa/specificity.js
 *
 * Calculates the specificity of CSS selectors
 * http://www.w3.org/TR/css3-selectors/#specificity
 *
 * Returns a selector integer value
 */

// The following regular expressions assume that selectors matching the preceding regular expressions have been removed
var attributeRegex = /(\[[^\]]+\])/g;
var idRegex = /(#[^\s\+>~\.\[:]+)/g;
var classRegex = /(\.[^\s\+>~\.\[:]+)/g;
var pseudoElementRegex = /(::[^\s\+>~\.\[:]+|:first-line|:first-letter|:before|:after)/g;
var pseudoClassRegex = /(:[^\s\+>~\.\[:]+)/g;
var elementRegex = /([^\s\+>~\.\[:]+)/g;
var notRegex = /:not\(([^\)]*)\)/g;
var ruleRegex = /\{[^]*/gm;
var separatorRegex = /[\*\s\+>~]/g;
var straysRegex = /[#\.]/g;

// Find matches for a regular expression in a string and push their details to parts
// Type is "a" for IDs, "b" for classes, attributes and pseudo-classes and "c" for elements and pseudo-elements
var findMatch = function(regex, type, types, selector) {
  var matches = selector.match(regex);
  if (matches) {
    for (var i = 0; i < matches.length; i++) {
      types[type]++;
      // Replace this simple selector with whitespace so it won't be counted in further simple selectors
      selector = selector.replace(matches[i], ' ');
    }
  }

  return selector;
}

// Calculate the specificity for a selector by dividing it into simple selectors and counting them
var calculate = function(selector) {
  var commaIndex = selector.indexOf(',');
  if (commaIndex !== -1) {
    selector = selector.substring(0, commaIndex);
  }

  var  types = {
    a: 0,
    b: 0,
    c: 0
  };

  // Remove the negation psuedo-class (:not) but leave its argument because specificity is calculated on its argument
  selector = selector.replace(notRegex, ' $1 ');

  // Remove anything after a left brace in case a user has pasted in a rule, not just a selector
  selector = selector.replace(ruleRegex, ' ');

  // Add attribute selectors to parts collection (type b)
  selector = findMatch(attributeRegex, 'b', types, selector);

  // Add ID selectors to parts collection (type a)
  selector = findMatch(idRegex, 'a', types, selector);

  // Add class selectors to parts collection (type b)
  selector = findMatch(classRegex, 'b', types, selector);

  // Add pseudo-element selectors to parts collection (type c)
  selector = findMatch(pseudoElementRegex, 'c', types, selector);

  // Add pseudo-class selectors to parts collection (type b)
  selector = findMatch(pseudoClassRegex, 'b', types, selector);

  // Remove universal selector and separator characters
  selector = selector.replace(separatorRegex, ' ');

  // Remove any stray dots or hashes which aren't attached to words
  // These may be present if the user is live-editing this selector
  selector = selector.replace(straysRegex, ' ');

  // The only things left should be element selectors (type c)
  findMatch(elementRegex, 'c', types, selector);

  return (types.a * 100) + (types.b * 10) + (types.c * 1);
}

var specificityCache = {};

exports.calculateSpecificity = function(selector) {
  var specificity = specificityCache[selector];
  if (specificity === undefined) {
    specificity = calculate(selector);
    specificityCache[selector] = specificity;
  }
  return specificity;
}

var validSelectorCache = {};
var testSelectorElement = null;

exports.isSelectorValid = function(selector) {
  var valid = validSelectorCache[selector];
  if (valid === undefined) {
    if (testSelectorElement == null) {
      testSelectorElement = document.createElement('div')
    }

    try {
      testSelectorElement.querySelector(selector);
      valid = true;
    } catch (error) {
      valid = false;
    }
    validSelectorCache[selector] = valid;
  }
  return valid;
}

exports.validateSelector = function(selector) {
  if (!exports.isSelectorValid(selector)) {
    var error = new SyntaxError(selector + ' is not a valid selector');
    error.code = 'EBADSELECTOR';
    throw error;
  }
}