angular/tools/manual_api_docs/blocks/defer.md

A type of block that can be used to defer load the JavaScript for components, directives and pipes used inside a component template.

Syntax

@defer ( on <trigger>; when <condition>; prefetch on <trigger>; prefetch when <condition> ) {
  <!-- deferred template fragment -->
  <calendar-cmp />
} @placeholder ( minimum? <duration> ) {
  <!-- placeholder template fragment -->
  <p>Placeholder</p>
} @loading ( minimum? <duration>; after? <duration> ) {
  <!-- loading template fragment -->
  <img alt="loading image" src="loading.gif" />
} @error {
  <!-- error template fragment -->
  <p>An loading error occurred</p>
}

Description

Blocks

Supported sections of a defer block. Note: only the @defer block template fragment is deferred loaded. The remaining optional blocks are eagerly loaded.

block	Description
@defer	The defer loaded block of content
@placeholder	Content shown prior to defer loading (Optional)
@loading	Content shown during defer loading (Optional)
@error	Content shown when defer loading errors occur (Optional)

Triggers

Triggers provide conditions for when defer loading occurs. Some allow a template reference variable as an optional parameter. Separate multiple triggers with a semicolon.

trigger	Triggers...
on idle	when the browser reports idle state (default)
on viewport(<elementRef>?)	when the element enters the viewport
on interaction(<elementRef>?)	when clicked, touched, or focused
on hover(<elementRef>?)	when element has been hovered
on immediate	when the page finishes rendering
on timer(<duration>)	after a specific timeout
when <condition>	on a custom condition

Prefetch

Configures prefetching of the defer block used in the @defer parameters, but does not affect rendering. Rendering is handled by the standard on and when conditions. Separate multiple prefetch configurations with a semicolon.

@defer (prefetch on <trigger>; prefetch when <condition>) {
  <!-- deferred template fragment -->
}

Learn more in the defer loading guide.