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164 lines
7.8 KiB
Markdown
164 lines
7.8 KiB
Markdown
[](https://codepen.io/bramus/pen/WNXyoYm)
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# Specificity
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`@bramus/specificity` is a package to calculate the specificity of CSS Selectors. It also includes some convenience functions to compare, sort, and filter an array of specificity values.
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Supports [Selectors Level 4](https://www.w3.org/TR/selectors-4/), including those special cases `:is()`, `:where()`, `:not()`, etc.
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Demo: [https://codepen.io/bramus/pen/WNXyoYm](https://codepen.io/bramus/pen/WNXyoYm)
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## Installation
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```bash
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npm i @bramus/specificity
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```
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## Usage / Example
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At its core, `@bramus/specificity` exposes a `Specificity` class. Its static `calculate` method can be used to calculate the specificity of a given CSS [Selector List](https://www.w3.org/TR/selectors-4/#grouping) string.
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```js
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import Specificity from '@bramus/specificity';
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const specificities = Specificity.calculate('header:where(#top) nav li:nth-child(2n), #doormat');
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```
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Because `calculate` accepts a [Selector List](https://www.w3.org/TR/selectors-4/#grouping) — which can contain more than 1 [Selector](https://www.w3.org/TR/selectors-4/#selector) — it will always return an array, with each entry being a `Specificity` instance — one per found selector.
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```js
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const specificities = Specificity.calculate('header:where(#top) nav li:nth-child(2n), #doormat');
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specificities.map((s) => s.toString()); // ~> ["(0,1,3)","(1,0,0)"]
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```
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💡 If you know you’re passing only a single Selector into `calculate()`, you can use JavaScript’s built-in destructuring to keep your variable names clean.
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```js
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const [s] = Specificity.calculate('header:where(#top) nav li:nth-child(2n)');
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s.toString(); // ~> "(0,1,3)"
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```
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💡 Under the hood, `@bramus/specificity` uses [CSSTree](https://github.com/csstree/csstree) to do the parsing of strings to Selectors. As a result, the `calculate` method also accepts a [CSSTree AST](https://github.com/csstree/csstree/blob/master/docs/ast.md) of the types `Selector` and `SelectorList`.
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If you have a pre-parsed CSSTree AST of the type `Selector` you can pass it into `Specificity.calculateForAST()`. It [performs slightly better](#benchmark) than `Specificity.calculate()` as it needs to check fewer things. It differs from `Specificity.calculate()` in that it does not return an array of `Specificity` instances but only a single value.
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## The Return Format
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A calculated specificity is represented as an instance of the `Specificity` class. The `Specificity` class includes methods to get the specificity value in a certain format, along with some convenience methods to compare it against other instances.
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```js
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// 🚀 Thunderbirds are go!
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import Specificity from '@bramus/specificity';
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// ✨ Calculate specificity for each Selector in the given Selector List
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const specificities = Specificity.calculate('header:where(#top) nav li:nth-child(2n), #doormat');
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// 🚚 The values in the array are instances of the Specificity class
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const s = specificities[0]; // Instance of Specificity
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// 👀 Read the specificity value using one of its accessors
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s.value; // { a: 0, b: 1, c: 3 }
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s.a; // 0
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s.b; // 1
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s.c; // 3
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// 🛠 Convert the calculated value to various formats using one of the toXXX() instance methods
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s.toString(); // "(0,1,3)"
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s.toArray(); // [0, 1, 3]
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s.toObject(); // { a: 0, b: 1, c: 3 }
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// 💡 Extract the matched selector string
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s.selectorString(); // "header:where(#top) nav li:nth-child(2n)"
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// 🔀 Use one of its instance comparison methods to compare it to another Specificity instance
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s.isEqualTo(specificities[1]); // false
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s.isGreaterThan(specificities[1]); // false
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s.isLessThan(specificities[1]); // true
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// 💻 Don’t worry about JSON.stringify()
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JSON.stringify(s);
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// {
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// "selector": 'header:where(#top) nav li:nth-child(2n)',
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// "asObject": { "a": 0, "b": 1, "c": 3 },
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// "asArray": [0, 1, 3],
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// "asString": "(0,1,3)",
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// }
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```
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## Utility Functions (Static Methods)
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This package also exposes some utility functions to work with specificities. These utility functions are all exposed as static methods on the `Specificity` class.
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- Comparing:
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- `Specificity.compare(s1, s2)`: Compares s1 to s2. Returns a value that can be:
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- `> 0` = Sort s2 before s1 _(i.e. s1 is more specific than s2)_
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- `0` = Keep original order of s1 and s2 _(i.e. s1 and s2 are equally specific)_
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- `< 0` = Sort s1 before s2 _(i.e. s1 is less specific than s2)_
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- `Specificity.equals(s1, s2)`: Returns `true` if s1 and s2 have the same specificity. If not, `false` is returned.
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- `Specificity.greaterThan(s1, s2)`: Returns `true` if s1 has a higher specificity than s2. If not, `false` is returned.
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- `Specificity.lessThan(s1, s2)`: Returns `true` if s1 has a lower specificity than s2. If not, `false` is returned.
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- Sorting:
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- `Specificity.sortAsc(s1, s2, …, sN)`: Sorts the given specificities in ascending order _(low specificity to high specificity)_
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- `Specificity.sortDesc(s1, s2, …, sN)`: Sorts the given specificities in descending order _(high specificity to low specificity)_
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- Filtering:
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- `Specificity.min(s1, s2, …, sN)`: Filters out the value with the lowest specificity
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- `Specificity.max(s1, s2, …, sN)`: Filters out the value with the highest specificity
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A specificity passed into any of these utility functions can be any of:
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- An instance of the included `Specificity` class
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- A simple Object such as `{'a': 1, 'b': 0, 'c': 2}`
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## Utility Functions (Standalone)
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All static methods the `Specificity` class exposes are also exported as standalone functions using [Subpath Exports](https://nodejs.org/api/packages.html#subpath-exports).
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If you're only interested in including some of these functions into your project you can import them from their Subpath. As a result, your bundle size will be reduced greatly _(except for including the standalone `calculate`, as it returns an array of `Specificity` instances that relies on the whole lot)_
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```js
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import { calculate, calculateForAST } from '@bramus/specificity/core';
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import { compare, equals, greaterThan, lessThan } from '@bramus/specificity/compare';
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import { min, max } from '@bramus/specificity/filter';
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import { sortAsc, sortDesc } from '@bramus/specificity/sort';
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```
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## Type Definitions
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Although `@bramus/specificity` is written in Vanilla JavaScript, it does include [Type Definitions](https://www.typescriptlang.org/docs/handbook/2/type-declarations.html) which are exposed via its `package.json`.
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## Binary/CLI
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`@bramus/specificity` exposes a binary named `specificity` to calculate the specificity of a given selector list on the CLI. For each selector that it finds, it'll print out the calculated specificity as a string on a new line.
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```bash
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$ specificity "header:where(#top) nav li:nth-child(2n), #doormat"
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(0,1,3)
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(1,0,0)
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```
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## Benchmark
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A benchmark is included, which you can invoke using `npm run benchmark`.
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Sample results (tested on a MacBook Air M3):
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```
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Specificity.calculate(string) x 420,682 ops/sec ±0.34% (98 runs sampled)
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Specificity.calculate(ast) - using SelectorList x 8,994,080 ops/sec ±0.25% (98 runs sampled)
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Specificity.calculate(ast) - using Selector x 11,054,856 ops/sec ±0.39% (91 runs sampled)
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Specificity.calculateForAST(ast) x 12,652,322 ops/sec ±0.35% (96 runs sampled)
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```
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## License
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`@bramus/specificity` is released under the MIT public license. See the enclosed `LICENSE` for details.
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## Acknowledgements
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The idea to create this package was sparked by [the wonderful Specificity Calculator created by Kilian Valkhof / Polypane](https://polypane.app/css-specificity-calculator/), a highly educational tool that not only calculates the specificity, but also explains which parts are responsible for it.
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The heavy lifting of doing the actual parsing of Selectors is done by [CSSTree](https://github.com/csstree/csstree).
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