evanw/esbuild · error · Error

Invalid footer file type

Error message

Invalid footer file type: ${type}

What it means

The `footer` option (build) mirrors `banner`: a map of file type to text, serialized as `--footer:TYPE=TEXT`. An '=' inside the TYPE breaks parsing, so esbuild rejects it at lib/shared/common.ts:328.

Solutions

1. Use a valid file type key without '=' (commonly 'js' or 'css').
2. Strip '=' from generated keys.

Example fix

// before
await build({ entryPoints: ['app.ts'], footer: { 'js=x': '/* trailer */' } })

// after
await build({ entryPoints: ['app.ts'], footer: { js: '/* trailer */' } })

const VALID_FOOTER_TYPES = new Set(['js', 'css'])
function assertFooterKeys(map) {
  for (const k of Object.keys(map ?? {})) {
    if (!VALID_FOOTER_TYPES.has(k)) throw new Error(`footer key must be 'js' or 'css', got ${k}`)
  }
}
assertFooterKeys(options.footer)

When it happens

Trigger: Passing `footer: { 'css=x': '...' }` or any footer key containing '='.

Common situations: Typo or generated key; using the wrong file-type identifier.

Related errors

• Expected in mangle cache to map to either a string or false
• Expected value for supported
• Expected value for to be a string, got instead
• Invalid banner file type
• Invalid charset

AI-assisted analysis of evanw/esbuild@f6058f8364 (2026-08-09). Data as JSON: /api/errors/cd6b82f1394e9f90. Report an issue: GitHub.

Appendix: source

Thrown at lib/shared/common.ts:328

  if (assetNames) flags.push(`--asset-names=${assetNames}`)
  if (mainFields) flags.push(`--main-fields=${validateAndJoinStringArray(mainFields, 'main field')}`)
  if (conditions) flags.push(`--conditions=${validateAndJoinStringArray(conditions, 'condition')}`)
  if (external) for (let name of external) flags.push(`--external:${validateStringValue(name, 'external')}`)
  if (alias) {
    for (let old in alias) {
      if (old.indexOf('=') >= 0) throw new Error(`Invalid package name in alias: ${old}`)
      flags.push(`--alias:${old}=${validateStringValue(alias[old], 'alias', old)}`)
    }
  }
  if (banner) {
    for (let type in banner) {
      if (type.indexOf('=') >= 0) throw new Error(`Invalid banner file type: ${type}`)
      flags.push(`--banner:${type}=${validateStringValue(banner[type], 'banner', type)}`)
    }
  }
  if (footer) {
    for (let type in footer) {
      if (type.indexOf('=') >= 0) throw new Error(`Invalid footer file type: ${type}`)
      flags.push(`--footer:${type}=${validateStringValue(footer[type], 'footer', type)}`)
    }
  }
  if (inject) for (let path of inject) flags.push(`--inject:${validateStringValue(path, 'inject')}`)
  if (loader) {
    for (let ext in loader) {
      if (ext.indexOf('=') >= 0) throw new Error(`Invalid loader extension: ${ext}`)
      flags.push(`--loader:${ext}=${validateStringValue(loader[ext], 'loader', ext)}`)
    }
  }
  if (outExtension) {
    for (let ext in outExtension) {
      if (ext.indexOf('=') >= 0) throw new Error(`Invalid out extension: ${ext}`)
      flags.push(`--out-extension:${ext}=${validateStringValue(outExtension[ext], 'out extension', ext)}`)
    }
  }

  if (entryPoints) {

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