A JavaScript module for calculating the specificity of CSS selectors
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Specificity Calculator

A JavaScript module for calculating and comparing the specificity of CSS selectors. The module is used on the Specificity Calculator website.

Specificity Calculator is built for CSS Selectors Level 3. Specificity Calculator isn’t a CSS validator. If you enter invalid selectors it will return incorrect results. For example, the negation pseudo-class may only take a simple selector as an argument. Using a psuedo-element or combinator as an argument for :not() is invalid CSS so Specificity Calculator will return incorrect results.

Supported runtime environments

The module is provided in two formats: an ECMAScript (ES) module in dist/specificity.mjs, and a Universal Module Definition (UMD) in dist/specificity.js. This enables support for the following runtime environments:

Browser

  • Directly loaded ES module
  • ES module in a precompiled script (using a bundler like Webpack or Rollup)
  • Global variable

Node.js

  • ES module
  • CommonJS module

Browser usage as a directly loaded ES module

<script type="module">
  import { calculate } from './specificity/dist/specificity.mjs';

  calculate('ul#nav li.active a');
</script>

Browser usage as an ES module in a precompiled script

Bundlers like Webpack and Rollup import from the module field in package.json, which is set to the ES module artefact, dist/specificity.mjs.

import { calculate } from 'specificity';

calculate('ul#nav li.active a');

Browser usage as a global variable

The UMD artefact, dist/specificity.js, sets a global variable, SPECIFICITY.

<script src="./specificity/dist/specificity.js"></script>

<script>
  SPECIFICITY.calculate('ul#nav li.active a');
</script>

Node.js usage as an ES module

The main field in package.json has an extensionless value, dist/specificity. This allows Node.js to use either the ES module, in dist/specificity.mjs, or the CommonJS module, in dist/specificity.js.

When Node.js is run with the --experimental-modules flag or an ES module loader, it will use the ES module artefact.

import { calculate } from 'specificity';

calculate('ul#nav li.active a');

Node.js usage as a CommonJS module

Otherwise, Node.js will use the UMD artefact, which contains a CommonJS module definition.

const { calculate } = require('specificity');

calculate('ul#nav li.active a');

Calculate function

The calculate function returns an array containing a result object for each selector input. Each result object has the following properties:

  • selector: the input
  • specificity: the result as a string e.g. 0,1,0,0
  • specificityArray: the result as an array of numbers e.g. [0, 1, 0, 0]
  • parts: array with details about each part of the selector that counts towards the specificity

Example

calculate('ul#nav li.active a');

/*
[
  {
    selector: 'ul#nav li.active a',
    specificity: '0,1,1,3',
    specificityArray: [0, 1, 1, 3],
    parts: [
      { selector: 'ul', type: 'c', index: 0, length: 2 },
      { selector: '#nav', type: 'a', index: 2, length: 4 },
      { selector: 'li', type: 'c', index: 5, length: 2 },
      { selector: '.active', type: 'b', index: 8, length: 7 },
      { selector: 'a', type: 'c', index: 13, length: 1 }
    ]
  }
]
*/

You can use comma separation to pass in multiple selectors:

calculate('ul#nav li.active a, body.ie7 .col_3 h2 ~ h2');

/*
[
  {
    selector: 'ul#nav li.active a',
    specificity: '0,1,1,3',
    ...
  },
  {
    selector: 'body.ie7 .col_3 h2 ~ h2',
    specificity: '0,0,2,3',
    ...
  }
]
*/

Comparing two selectors

Specificity Calculator also exports a compare function. This function accepts two CSS selectors or specificity arrays, a and b.

  • It returns -1 if a has a lower specificity than b
  • It returns 1 if a has a higher specificity than b
  • It returns 0 if a has the same specificity than b
compare('div', '.active');            // -1
compare('#main', 'div');              // 1
compare('span', 'div');               // 0
compare('span', [0, 0, 0, 1]);        // 0
compare('#main > div', [0, 1, 0, 1]); // 0

Ordering an array of selectors by specificity

You can pass the compare function to Array.prototype.sort to sort an array of CSS selectors by specificity.

import { compare } from 'specificity';

['#main', 'p', '.active'].sort(compare); // ['p', '.active', '#main']

Command-line usage

Run npm install specificity to install the module locally, or npm install -g specificity for global installation. Run specificity without arguments to learn about its usage:

$ specificity
Usage: specificity <selector>
Computes specificity of a CSS selector.

Pass a selector as the first argument to get its specificity computed:

$ specificity "ul#nav li.active a"
0,1,1,3

Testing

To install dependencies, run: npm install

Then to test, run: npm test