What causes "unsupported target: %v" in gohugoio/hugo?

Use only supported values: es5, es6/es2015, es2016 through es2024, esnext, or (e.g. chrome80).

unsupported target

Error message

unsupported target: %v

What it means

Thrown when the Target slice is non-empty but none of its values match either a known ES language version (es5, esnext, etc.) nor any known engine prefix. The result is a wholly unrecognized target list.

Solutions

Use only supported values: es5, es6/es2015, es2016 through es2024, esnext, or <engine><version> (e.g. chrome80).

Example fix

// before
{{ $opts := dict "targets" (slice "modern") }}

// after
{{ $opts := dict "targets" (slice "es2020") }}

Defensive patterns

Strategy: validation

Validate before calling

allowed := map[string]bool{"es5": true, "es6": true, "es2015": true, "es2016": true, "es2017": true, "es2018": true, "es2019": true, "es2020": true, "es2021": true, "es2022": true, "es2023": true, "es2024": true, "esnext": true, "": true}
engineRe := regexp.MustCompile(`^(chrome|deno|edge|firefox|hermes|ie|ios|node|opera|rhino|safari)\d+$`)
for _, t := range opts.Target {
    if !allowed[t] && !engineRe.MatchString(t) {
        return fmt.Errorf("unsupported target: %q", t)
    }
}

Prevention

Maintain a whitelist of accepted target strings in your config layer.
Lowercase Target values before validation (Hugo does this internally).
Reject unknown targets at config load rather than at build time.

When it happens

Trigger: Setting Target to ["web"], ["modern"], or any string not present in nameTarget and not starting with a known engine name.

Common situations: User invents target names like "modern" or "legacy"; typo like es2200; expects case-variants (DecodeExternalOptions lowercases Target, but a value with no lowercase mapping like "MODERN" still won't resolve).

Related errors

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/234df0970d37ce85. Report an issue: GitHub.

Appendix: source

Thrown at internal/js/esbuild/options.go:296

	var engines []api.Engine

OUTER:
	for _, value := range opts.Target {
		for _, engine := range engineKeysSorted {
			if strings.HasPrefix(value, engine) {
				version := value[len(engine):]
				if version == "" {
					return fmt.Errorf("invalid engine version: %q", value)
				}
				engines = append(engines, api.Engine{Name: engineName[engine], Version: version})
				continue OUTER
			}
		}
	}

	if target == 0 && len(engines) == 0 && len(opts.Target) > 0 {
		return fmt.Errorf("unsupported target: %v", opts.Target)
	}

	if target == 0 {
		target = api.ESNext
	}

	var loaders map[string]api.Loader
	if opts.IsCSS {
		loaders = make(map[string]api.Loader)
		// Add default CSS file loaders.
		// May be overridden by opts.Loaders.
		maps.Copy(loaders, extensionToLoaderMapCSS)
	}
	if opts.Loaders != nil {
		if loaders == nil {
			loaders = make(map[string]api.Loader)
		}

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