evanw/esbuild · error · Error

Invalid out extension: ${ext}

Error message

Invalid out extension: ${ext}

What it means

Error "Invalid out extension: ${ext}" thrown in evanw/esbuild.

Source

Thrown at lib/shared/common.ts:339

      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) {
    if (Array.isArray(entryPoints)) {
      for (let i = 0, n = entryPoints.length; i < n; i++) {
        let entryPoint = entryPoints[i]
        if (typeof entryPoint === 'object' && entryPoint !== null) {
          let entryPointKeys: OptionKeys = Object.create(null)
          let input = getFlag(entryPoint, entryPointKeys, 'in', mustBeString)
          let output = getFlag(entryPoint, entryPointKeys, 'out', mustBeString)
          checkForInvalidFlags(entryPoint, entryPointKeys, 'in entry point at index ' + i)
          if (input === undefined) throw new Error('Missing property "in" for entry point at index ' + i)
          if (output === undefined) throw new Error('Missing property "out" for entry point at index ' + i)
          entries.push([output, input])
        } else {
          entries.push(['', validateStringValue(entryPoint, 'entry point at index ' + i)])

View on GitHub (pinned to 6ff1d8b0d8)

Solutions

1. Ensure out extension keys are valid file extensions beginning with a dot (e.g. ".js", ".css")
2. Fix or remove the malformed extension in the outExtension map

Example fix

esbuild.buildSync({ outExtension: { '.js': '.mjs' } });

When it happens

Trigger: Thrown at lib/shared/common.ts:339 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of evanw/esbuild@6ff1d8b0d8 (2026-08-03). Data as JSON: /data/errors/40fdcfa32bd57559.json. Report an issue: GitHub.