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# nth-check [![Build Status](](

A performant nth-check parser & compiler.

### 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](

### API

var nthCheck = require("nth-check");

##### `nthCheck(formula)`

First parses, then compiles the formula.

##### `nthCheck.parse(formula)`

Parses the expression, throws a `SyntaxError` if it fails, otherwise returns an array containing two elements.


nthCheck.parse("2n+3") //[2, 3]

##### `nthCheck.compile([a, b])`

Takes an array with two elements (as returned by `.parse`) and returns a highly optimized function.

If the formula doesn't match any elements, it returns [`boolbase`]('s `falseFunc`, otherwise, a function accepting an _index_ is returned, which returns whether or not a passed _index_ matches the formula. (Note: The spec starts counting at `1`, the returned function at `0`).

var check = nthCheck.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](
Tidelift will coordinate the fix and disclosure.

## `nth-check` for enterprise

Available as part of the Tidelift Subscription

The maintainers of `nth-check` and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. [Learn more.](