Document templates.Defer

content/en/functions/templates/Defer.md

@@ -0,0 +1,81 @@
+---
+title: templates.Defer
+description: Defer execution of a template until after all sites and output formats have been rendered.
+categories: []
+keywords: []
+toc: true
+action:
+  aliases: []
+  related: []
+  returnType: string
+  signatures: [templates.Defer OPTIONS]
+aliases: [/functions/templates.defer]
+---
+
+{{< new-in "0.128.0" >}}
+
+{{% note %}}
+This function only works in combination with the `with` keyword.
+{{% /note %}}
+
+In some rare use cases, you may need defer the execution of a template until after all sites and output formats have been rendered. One such example could be [TailwindCSS](https://github.com/bep/hugo-starter-tailwind-basic) using the output of [hugo_stats.json](https://gohugo.io/getting-started/configuration/#configure-build) to determine which classes and other HTML identifiers are being used in the final output:
+
+```go-html-template
+{{ with (templates.Defer (dict "key" "styles" )) }}
+      {{ $options := dict "inlineImports" true }}
+      {{ $styles := resources.Get "css/styles.css" }}
+      {{ $styles = $styles | resources.PostCSS $options }}
+      {{ if hugo.IsProduction }}
+        {{ $styles = $styles | minify | fingerprint }}
+      {{ end }}
+      <link href="{{ $styles.RelPermalink }}" rel="stylesheet" />
+{{ end }}
+```
+
+
+{{% note %}}
+Variables defined on the outside are not visible on the inside and vice versa.
+{{% /note %}}
+
+For the above to work well when running the server (or `hugo -w`), you want to have a configuration similar to this:
+
+{{< code-toggle file=hugo >}}
+[module]
+[[module.mounts]]
+source       = "hugo_stats.json"
+target       = "assets/notwatching/hugo_stats.json"
+disableWatch = true
+[build]
+writeStats           = true
+[[build.cachebusters]]
+source = "assets/notwatching/hugo_stats\\.json"
+target = "styles\\.css"
+[[build.cachebusters]]
+source = "(postcss|tailwind)\\.config\\.js"
+target = "css"
+{{< /code-toggle >}}
+
+## Options
+
+The `templates.Defer` function takes a single argument, a map with the following optional keys:
+
+key (`string`)
+: The key to use for the deferred template. This will, combined with a hash of the template content, be used as a cache key. If this is not set, Hugo will execute the deferred template on every render. This is not what you want for shared resources like CSS and JavaScript.
+
+data (`map`) 
+: Optional map to pass as data the deferred template. This will be available in the deferred template as `.` or `$`.
+
+
+```go-html-template
+Language Outside: {{ site.Language.Lang }}
+Page Outside: {{ .RelPermalink }}
+I18n: {{ i18n "hello" }}
+{{ $data := (dict "page" . )}}
+{{ with (templates.Defer (dict "data" $data )) }}
+     Language Inside: {{ site.Language.Lang }}
+     Page Inside: {{ .page.RelPermalink }}
+     I18n: {{ i18n "hello" }}
+{{ end }}
+```
+
+The [Output Format](/templates/output-formats/), [Site](/methods/page/site/), and [language](/methods/site/language) will be the same, even if the execution is deferred. In the example above, this means that the `site.Language.Lang` and `.RelPermalink` will be the same inside and outside the deferred template.