What causes "Multiple selector arguments not supported here" in denoland/deno?

Keep a single selector in the of-clause: ":nth-child(1 of a)".

denoland/deno · error · Error

Multiple selector arguments not supported here

Error message

Multiple selector arguments not supported here

What it means

Inside the `of <selector>` clause of :nth-child(), the parser sets throwOnComma = true (cli/js/40_lint_selector.js). A comma encountered while that flag is set throws `Multiple selector arguments not supported here` — the An+B-of form accepts exactly one selector argument, unlike :is()/:has()/:not() which take selector lists. The flag stays set until the next closing parenthesis, so any comma between `of` and the ')' that closes the :nth-child throws.

Solutions

Keep a single selector in the of-clause: ":nth-child(1 of a)".
List multiple selector keys — top-level comma lists are allowed because visitor keys are split before parsing: "Var:nth-child(1 of a), Var:nth-child(1 of b)".
Alternatively, match the parent/child relationship in the key and verify the sibling index in JavaScript inside the visitor.

Example fix

// before
"Var:nth-child(1 of A, B)"(node) { /* ... */ },

// after
"Var:nth-child(1 of A), Var:nth-child(1 of B)"(node) { /* ... */ },

Defensive patterns

Strategy: validation

Validate before calling

function checkOfClause(key) {
  const m = key.match(/:nth-child\([^)]*\bof\b[^)]*,/);
  if (m) {
    throw new Error(`Selector list inside 'of' not supported in "${key}"; split into multiple keys`);
  }
}

Prevention

Keep exactly one selector inside an of-clause.
Express alternatives as a top-level comma list in the visitor key — those are split before parsing and are safe.
Do not try to sneak a list in via :is() inside the of-clause; commas there hit the same guard.

When it happens

Trigger: Visitor keys like "Var:nth-child(1 of a, b)" — a comma-separated selector list inside the of-clause. Note the workaround of nesting (:nth-child(1 of :is(a, b))) also throws for the same reason, because the comma arrives while throwOnComma is still true.

Common situations: Trying to count only among several sibling types at once, CSS-style; extending an existing of-clause with a second selector during iteration.

Related errors

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/ce1eb19acef83f09. Report an issue: GitHub.

Appendix: source

Thrown at cli/js/40_lint_selector.js:729

        case "not": {
          lex.next();
          lex.expect(Token.BraceOpen);
          lex.next();

          current.push({
            type: PSEUDO_NOT,
            selectors: [],
          });
          stack.push([]);

          continue;
        }
        default:
          throw new Error(`Unknown pseudo selector: '${lex.value}'`);
      }
    } else if (lex.token === Token.Comma) {
      if (throwOnComma) {
        throw new Error(`Multiple selector arguments not supported here`);
      }

      lex.next();
      if (lex.token === Token.Space) {
        lex.next();
      }

      popSelector(result, stack);
      stack.push([]);
      continue;
    } else if (lex.token === Token.BraceClose) {
      throwOnComma = false;
      popSelector(result, stack);
    } else if (lex.token === Token.Op) {
      current.push({
        type: RELATION_NODE,
        op: getAttrOp(lex.value),
      });

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