denoland/deno · error

Missing parameter name at {}

Error message

Missing parameter name at {}

What it means

In the same lexer (cli/lsp/path_to_regex.rs:125), after an opening '{' the parser reads [0-9A-Za-z_] characters as the parameter name. If none are consumed the name is empty and it fails with "Missing parameter name at {index}" — the schema used a parameter placeholder with no name, like "{}" or "{$}".

Source

Thrown at cli/lsp/path_to_regex.rs:125

        });
        index += 1;
      }
      Some(':') => {
        let mut name = String::new();
        while let Some(c) = chars.peek() {
          if (*c >= '0' && *c <= '9')
            || (*c >= 'A' && *c <= 'Z')
            || (*c >= 'a' && *c <= 'z')
            || *c == '_'
          {
            let ch = chars.next().unwrap();
            name.push(ch);
          } else {
            break;
          }
        }
        if name.is_empty() {
          return Err(anyhow!("Missing parameter name at {}", index));
        }
        let name_len = name.len();
        tokens.push(LexToken {
          token_type: TokenType::Name,
          index,
          value: name,
        });
        index += 1 + name_len;
      }
      Some('(') => {
        let mut count = 1;
        let mut pattern = String::new();

        if chars.peek() == Some(&'?') {
          return Err(anyhow!(
            "Pattern cannot start with \"?\" at {}.",
            index + 1
          ));

View on GitHub (pinned to 89f33cbef2)

Solutions

1. Name the parameter: "/x/{package}/mod.ts"
2. If the braces are literal, escape them or remove them from the schema
3. Ensure any templating that fills {name} placeholders never emits an empty name

Example fix

// before
{ "schema": "/x/{}/mod.ts" }

// after
{ "schema": "/x/{package}/mod.ts" }

function hasEmptyPlaceholder(s: string): boolean {
  return /\{[^A-Za-z0-9_}\s]/.test(s) || /\{\}/.test(s);
}

When it happens

Trigger: A schema containing an empty placeholder: "/x/{}/mod.ts"; a '{' immediately followed by a non-identifier character like '-' or '.'; unbalanced braces from a broken template render.

Common situations: Template engines substituting an empty variable into {var}; copy-paste of path-to-regexp v6 syntax into a config that expects named groups; typos like {package } with a space (space is not an identifier char, so name stays empty).

Related errors

• Unexpected end of string at {}.
• Unexpected termination of string.
• Pattern cannot start with "?" at {}.
• Capturing groups are not allowed at {}.
• Unbalanced pattern at {}.

AI-assisted analysis of denoland/deno@89f33cbef2 (2026-08-16). Data as JSON: /api/errors/93417657e010ea8e. Report an issue: GitHub.