@for
The @for block repeatedly renders content of a block for each item in a collection.
Description
The @for block repeatedly renders content of a block for each item in a collection.
Syntax
@for (item of items; track item.name) {<li>{{ item.name }}</li>} @empty {<li>There are no items.</li>}Description
The @for block renders its content in response to changes in a collection. Collections can be any
JavaScript iterable,
but there are performance advantages of using a regular Array.
You can optionally include an @empty section immediately after the @for block content. The
content of the @empty block displays when there are no items.
Angular's @for block does not support flow-modifying statements like JavaScript's continue or break.
track and objects identity
The value of the track expression determines a key used to associate array items with the views in
the DOM. Having clear indication of the item identity allows Angular to execute a minimal set of DOM
operations as items are added, removed or moved in a collection.
To optimize performance, especially in loops over immutable data, ensure the track expression is effectively used to
identify each item uniquely. Because of the potential for poor performance, the track expression
is required for the @for loops.
For collections that remain static , track $index provides a straightforward tracking mechanism. For dynamic
collections experiencing additions, deletions, or reordering, opt for a
unique property of each item as the tracking key.
Track expressions can only reference $index, the item, and fields from the component class. If the let segment of the @for block introduced an alias for $index, that alias may also be referenced.
$index and other contextual variables
Inside @for contents, several implicit variables are always available:
| Variable | Meaning |
|---|---|
$count |
Number of items in a collection iterated over |
$index |
Index of the current row |
$first |
Whether the current row is the first row |
$last |
Whether the current row is the last row |
$even |
Whether the current row index is even |
$odd |
Whether the current row index is odd |
These variables are always available with these names, but can be aliased via a let segment:
@for (item of items; track item.id; let idx = $index, e = $even) {Item #{{ idx }}: {{ item.name }}}The aliasing is especially useful in case of using nested @for blocks where contextual variable
names could collide.