# nth-check

> Parses and compiles CSS nth-checks to highly optimized functions.

Latest version **3.0.1** (published 2026-03-17) · BSD-2-Clause license · 0 weekly downloads

## Install

```sh
npm install nth-check
pnpm add nth-check
yarn add nth-check
bun add nth-check
```

## Health

**Score 70/100 (B)** — status: active.

Positive: has types; esm support; no vulnerabilities; has provenance; high maintenance score; high quality score.

Warnings: low downloads.

## Facts

| | |
|---|---|
| Version | 3.0.1 |
| Published | 2026-03-17 |
| First published | 2014-02-15 |
| Weekly downloads | 0 |
| License | BSD-2-Clause |
| TypeScript types | bundled |
| Module format | ESM + CommonJS |
| Node | >=20.19.0 |
| Dependencies | 1 |
| Unpacked size | 30.8 KB |
| Known vulnerabilities | 0 |
| Install scripts | no |
| Provenance | attested (GitHub Actions) |
| GitHub stars | 63 |
| Author | Felix Boehm |
| Maintainers | fb55 |
| Keywords | nth-child, nth, css |

## Links

- npm: https://www.npmjs.com/package/nth-check
- Repository: https://github.com/fb55/nth-check
- Issues: https://github.com/fb55/nth-check/issues
- Funding: https://github.com/fb55/nth-check?sponsor=1
- npm.io page: https://npm.io/package/nth-check

## Dependencies (1)

- [boolbase](https://npm.io/package/boolbase.md) ^2.0.0

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## Recent versions

- 3.0.1 (latest) — 2026-03-17
- 3.0.0 — 2026-03-17
- 2.1.1 — 2022-05-23
- 2.1.0 — 2022-05-22
- 2.0.1 — 2021-09-16
- 2.0.0 — 2020-12-01
- 1.0.2 — 2018-10-21
- 1.0.1 — 2015-03-12
- 1.0.0 — 2014-02-15

## README

# nth-check [![Build Status](https://travis-ci.org/fb55/nth-check.svg)](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()`. It can be used to check if a given index matches a given nth-rule, or to generate a sequence of indices matching a given nth-rule.

`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`
```

##### `generate([a, b])`

Returns a function that produces a monotonously increasing sequence of indices.

If the sequence has an end, the returned function will return `null` after the last index in the sequence.

**Example:** An always increasing sequence

```js
const gen = nthCheck.generate([2, 3]);

gen(); // `1`
gen(); // `3`
gen(); // `5`
gen(); // `8`
gen(); // `11`
```

**Example:** With an end value

```js
const gen = nthCheck.generate([-2, 5]);

gen(); // 0
gen(); // 2
gen(); // 4
gen(); // null
```

##### `sequence(formula)`

Parses and compiles a formula to a generator that produces a sequence of indices. Combination of `parse` and `generate`.

**Example:** An always increasing sequence

```js
const gen = nthCheck.sequence("2n+3");

gen(); // `1`
gen(); // `3`
gen(); // `5`
gen(); // `8`
gen(); // `11`
```

**Example:** With an end value

```js
const gen = nthCheck.sequence("-2n+5");

gen(); // 0
gen(); // 2
gen(); // 4
gen(); // null
```

---

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.

---
_Source: https://npm.io/package/nth-check · Machine-readable twin of the npm.io package page. Health data is recomputed on every publish._