# nth-check [](https://travis-ci.org/fb55/nth-check)
Parses and compiles CSS nth-checks to highly optimized functions.
### About
This module can be used to parse & compile nth-checks, as they are found in CSS 3's `nth-child()` and `nth-last-of-type()`.
`nth-check` focusses on speed, providing optimized functions for different kinds of nth-child formulas, while still following the [spec](http://www.w3.org/TR/css3-selectors/#nth-child-pseudo).
### API
```js
import nthCheck, { parse, compile } from "nth-check";
```
##### `nthCheck(formula)`
Parses and compiles a formula to a highly optimized function. Combination of `parse` and `compile`.
If the formula doesn't match any elements, it returns [`boolbase`](https://github.com/fb55/boolbase)'s `falseFunc`. Otherwise, a function accepting an _index_ is returned, which returns whether or not the passed _index_ matches the formula.
**Note**: The nth-rule starts counting at `1`, the returned function at `0`.
**Example:**
```js
const check = nthCheck("2n+3");
check(0); // `false`
check(1); // `false`
check(2); // `true`
check(3); // `false`
check(4); // `true`
check(5); // `false`
check(6); // `true`
```
##### `parse(formula)`
Parses the expression, throws an `Error` if it fails. Otherwise, returns an array containing the integer step size and the integer offset of the nth rule.
**Example:**
```js
parse("2n+3"); // [2, 3]
```
##### `compile([a, b])`
Takes an array with two elements (as returned by `.parse`) and returns a highly optimized function.
**Example:**
```js
const check = compile([2, 3]);
check(0); // `false`
check(1); // `false`
check(2); // `true`
check(3); // `false`
check(4); // `true`
check(5); // `false`
check(6); // `true`
```
---
License: BSD-2-Clause
## Security contact information
To report a security vulnerability, please use the [Tidelift security contact](https://tidelift.com/security).
Tidelift will coordinate the fix and disclosure.
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