swc-project/swc · error · Error

Invalid An+B microsyntax

Error message

Invalid An+B microsyntax

What it means

Fired while parsing the An+B microsyntax (e.g. in :nth-child()): a dimension token representing the `n` part was read, but its unit did not start with the character 'n'/'N', so it is not an <n-dimension>. This is a validation guard on the dimension's unit; input at fault is something like `:nth-child(3x)` instead of `:nth-child(3n)`.

Source

Thrown at crates/swc_css_parser/src/parser/selectors/mod.rs:1308

                                unreachable!();
                            }
                        };

                        let n_char = dimension.2.chars().next();

                        if n_char != Some('n') && n_char != Some('N') {
                            return Err(Error::new(
                                span,
                                ErrorKind::Expected("'n' character in Ident"),
                            ));
                        }

                        a = Some(dimension.0 as i32);
                        a_raw = Some(dimension.1);
                        n_value =  (*dimension.2).to_string();
                    }
                    _ => {
                        return Err(Error::new(span, ErrorKind::InvalidAnPlusBMicrosyntax));
                    }
                };

                self.input.skip_ws();

                let mut b = None;
                let mut b_raw = None;

                let dash_after_n = n_value.chars().nth(1);

                match cur!(self) {
                    // '+'? n <signed-integer>
                    // -n <signed-integer>
                    // <n-dimension> <signed-integer>
                    tok!("number") if dash_after_n.is_none() => {
                        let number = match bump!(self) {
                            Token::Number { value, raw, .. } => (value, raw),
                            _ => {

View on GitHub (pinned to 5176682b65)

Solutions

1. Follow `n` with `+`/`-` and a signless integer, or nothing at all
2. Report the collected error
3. Fix the selector

When it happens

Trigger: Thrown at crates/swc_css_parser/src/parser/selectors/mod.rs:1308 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/3ccf893636349235. Report an issue: GitHub.