parcel-bundler/parcel · error · ThrowableDiagnostic

Unknown template language: "${template.lang}"

Error message

Unknown template language: "${template.lang}"

What it means

Thrown by @parcel/transformer-vue when a <template lang="..."> uses a language other than html/htm and consolidate has no matching preprocessor (consolidate[template.lang] is undefined). The transformer looks up the renderer in the consolidate library to convert the template source to HTML before compileTemplate runs.

Source

Thrown at packages/transformers/vue/src/VueTransformer.js:238

          await options.inputFS.readFile(
            await resolve(asset.filePath, template.src),
          )
        ).toString();
        template.lang = extname(template.src).slice(1);
      }
      let content = template.content;
      if (template.lang && !['htm', 'html'].includes(template.lang)) {
        let options = {};
        let preprocessor = consolidate[template.lang];
        // Pug doctype fix (fixes #7756)
        switch (template.lang) {
          case 'pug':
            options.doctype = 'html';
            break;
        }
        if (!preprocessor) {
          // TODO: codeframe
          throw new ThrowableDiagnostic({
            diagnostic: {
              message: md`Unknown template language: "${template.lang}"`,
              origin: '@parcel/transformer-vue',
            },
          });
        }
        content = await preprocessor.render(content, options);
      }
      let templateComp = compiler.compileTemplate({
        filename: asset.filePath,
        source: content,
        inMap: template.src ? undefined : template.map,
        scoped: styles.some(style => style.scoped),
        isFunctional,
        compilerOptions: {
          ...config.compilerOptions,
          bindingMetadata: script ? script.bindings : undefined,
        },

View on GitHub (pinned to 59484858a1)

Solutions

1. Install the missing template engine: `npm install pug` (and ensure @parcel/transformer-vue depends on consolidate).
2. Correct the lang attribute to a supported consolidate engine name.
3. Drop the lang attribute to use plain HTML if no preprocessor is needed.

Example fix

// before
<template lang="pug">div</template>  <!-- pug not installed -->
// after
<template lang="pug">div</template>  <!-- after `npm i pug` -->
// or
<template><div/></template>

import consolidate from 'consolidate';
const lang = template.lang;
if (lang && !['htm','html'].includes(lang) && !consolidate[lang]) {
  throw new Error(`Template lang '${lang}' has no consolidate preprocessor; install it or remove the lang attribute.`);
}

function isSupportedTemplateLang(lang) {
  return !lang || ['htm','html'].includes(lang) || Boolean(consolidate[lang]);
}

When it happens

Trigger: template.lang is set to a value (pug, handlebars, ejs, etc.) but the corresponding consolidate engine — or its underlying npm package — is not installed, so consolidate[lang] is undefined.

Common situations: Using `<template lang="pug">` without installing pug, or a lang value consolidate doesn't ship a key for (typo like 'pugjs' or a niche engine not bundled). Removing a previously-installed preprocessor breaks builds retroactively.

Related errors

• [dynamic diagnostic from Vue template compiler errors]
• Vue config should be an object.
• [dynamic diagnostic from Vue compiler parse errors]
• Unknown script language: "${script.lang}"
• Unknown style language: "${style.lang}"

AI-assisted analysis of parcel-bundler/parcel@59484858a1 (2026-08-13). Data as JSON: /api/errors/da3b64aa553c4585. Report an issue: GitHub.