denoland/deno · error

Unsupported media type for snapshotting {media_type:?} for f

Error message

Unsupported media type for snapshotting {media_type:?} for file {}

What it means

When preparing startup snapshot inputs, runtime/transpile.rs accepts only TypeScript, JavaScript, and Mjs sources. Any other media type (JSX, TSX, JSON, CSS, etc.) reaches a panic because the snapshot transpile path has no emitter for it.

Source

Thrown at runtime/transpile.rs:87

  let source = maybe_substitute_node_version(&name_string, source);
  // Always transpile `node:` built-in modules, since they might be TypeScript.
  let media_type = if name.starts_with("node:") {
    MediaType::TypeScript
  } else {
    MediaType::from_path(Path::new(&name))
  };

  match media_type {
    MediaType::TypeScript => {}
    MediaType::JavaScript | MediaType::Mjs => {
      if minify {
        let source =
          minify_source_with_rolldown(&name_string, source.as_ref())?;
        return Ok((source.into(), None));
      }
      return Ok((source, None));
    }
    _ => panic!(
      "Unsupported media type for snapshotting {media_type:?} for file {}",
      name
    ),
  }

  let parsed = deno_ast::parse_module(ParseParams {
    specifier: deno_core::url::Url::parse(&name).unwrap(),
    text: source.into(),
    media_type,
    capture_tokens: false,
    scope_analysis: false,
    maybe_syntax: None,
  })
  .map_err(|e| JsErrorBox::from_err(JsParseDiagnostic(e)))?;
  let transpiled_source = parsed
    .transpile(
      &deno_ast::TranspileOptions {
        imports_not_used_as_values: deno_ast::ImportsNotUsedAsValues::Remove,

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Filter snapshot inputs to .js/.mjs/.ts before invoking the transpile step
2. Pre-transpile JSX/TSX to plain JS yourself if those files must be snapshotted
3. Point the snapshot build at an explicit file list instead of a directory glob

Example fix

// before
let files = std::fs::read_dir(dir)?; // picks up app.jsx -> panic

// after
let files = std::fs::read_dir(dir)?.filter(|e| {
  matches!(
    e.as_ref().ok().and_then(|e| e.path().extension().and_then(|s| s.to_str())),
    Some("js") | Some("mjs") | Some("ts")
  )
});

fn snapshottable(path: &std::path::Path) -> bool {
  matches!(
    path.extension().and_then(|s| s.to_str()),
    Some("js") | Some("mjs") | Some("ts")
  )
}

for path in inputs {
  if !snapshottable(&path) {
    return Err(format!("{} is not snapshottable; pre-transpile or exclude it", path.display()));
  }
}

When it happens

Trigger: Feeding a .jsx, .tsx, .json, .css, or other non-JS/TS file into snapshot/transpile input - e.g. a custom snapshot build script that globs a directory containing mixed file types.

Common situations: Custom embedders building snapshots with broad file globs; workspaces where JSX files sit next to JS; bundler outputs containing unexpected extensions.

Related errors

• Unexpected media type {media_type} for {specifier}
• Attempted to read snapshot data out of range: {id} (of {})
• Attempted to read the snapshot data at index {id} twice
• Invalid data type at index {id}, expected '{}'
• Bootstrap exception: {error}

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/631bba96a3666daa. Report an issue: GitHub.