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.
API
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'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:
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:
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:
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
const gen = nthCheck.generate([2, 3]);
gen(); // `1`
gen(); // `3`
gen(); // `5`
gen(); // `8`
gen(); // `11`
Example: With an end value
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
const gen = nthCheck.sequence("2n+3");
gen(); // `1`
gen(); // `3`
gen(); // `5`
gen(); // `8`
gen(); // `11`
Example: With an end value
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. Tidelift will coordinate the fix and disclosure.