Error Handling

Error handling in nth-check primarily occurs during the parsing stage of CSS nth-child formulas. The module distinguishes between syntax errors, which trigger exceptions, and logical edge cases (formulas that match no elements), which are handled by returning specialized optimized functions.

The system ensures that invalid formulas are caught early during the parse phase, while the compile phase handles mathematical edge cases where a formula might be logically valid but results in an empty set of matches.

Parsing Validation

The parse function is the primary gatekeeper for input validation. It performs a manual scan of the formula string and throws a standard Error if the input does not conform to the expected CSS nth-rule syntax.

Syntax Verification Logic

Parsing involves checking for valid signs, integers, and the presence of the n character. The parser throws an error in two specific scenarios:

1. Missing Numeric Component: When a number is expected but not found (e.g., after a sign or at the end of the string).
2. Trailing Characters: When there is leftover content in the string after the valid formula has been processed.

Validation Examples

The following table summarizes common invalid inputs that trigger parser errors:

Sources: src/fixtures/rules.ts:39-50, src/parse.spec.ts:7-11

Compiler Logical Handling

Unlike the parser, the compile function does not throw errors for validly parsed tuples. Instead, it handles logical "non-matches" by returning optimized utility functions from the boolbase library.

Empty Match Sets

If a formula is mathematically impossible to satisfy (e.g., both a and b resulting in indices less than 1), the system avoids runtime errors during index checking by returning a falseFunc.

Implementation of Empty Sequence

For the generate and sequence features, error handling manifests as returning null when a sequence is exhausted or logically empty.

• Non-positive a values: If a === 0 and the offset b is less than 0, the generator immediately returns a function that always yields null.
• Negative a values: The generator calculates a minValue and returns null once the sequence exceeds the upper bound b.

Sources: src/compile.ts:89-106, src/compile.spec.ts:79-88

Summary of Error States

Sources: src/parse.ts:54-56, src/compile.ts:37, src/compile.spec.ts:86-90

Error handling in nth-check ensures that developers receive immediate feedback via exceptions during development when providing malformed CSS selectors, while maintaining high performance at runtime by returning constant boolean functions for logically empty sets.