NgFor directive instantiates a template once per item from an iterable. The context for
each instantiated template inherits from the outer context with the given loop variable set
to the current item from the iterable.
NgFor provides several exported values that can be aliased to local variables:
index will be set to the current loop iteration for each template context.
first will be set to a boolean value indicating whether the item is the first one in the
last will be set to a boolean value indicating whether the item is the last one in the
even will be set to a boolean value indicating whether this item has an even index.
odd will be set to a boolean value indicating whether this item has an odd index.
When the contents of the iterator changes,
NgFor makes the corresponding changes to the DOM:
- When an item is added, a new instance of the template is added to the DOM.
- When an item is removed, its template instance is removed from the DOM.
- When items are reordered, their respective templates are reordered in the DOM.
- Otherwise, the DOM element for that item will remain the same.
Angular uses object identity to track insertions and deletions within the iterator and reproduce
those changes in the DOM. This has important implications for animations and any stateful
<input> elements which accept user input) that are present. Inserted rows can be
animated in, deleted rows can be animated out, and unchanged rows retain any unsaved state such
as user input.
It is possible for the identities of elements in the iterator to change while the data does not.
This can happen, for example, if the iterator produced from an RPC to the server, and that
RPC is re-run. Even if the data hasn't changed, the second response will produce objects with
different identities, and Angular will tear down the entire DOM and rebuild it (as if all old
elements were deleted and all new elements inserted). This is an expensive operation and should
be avoided if possible.
To customize the default tracking algorithm,
trackBy takes a function which has two arguments:
trackBy is given, Angular tracks changes by the return value of the function.
<li *ngFor="let item of items; let i = index; trackBy: trackByFn">...</li>
<li template="ngFor let item of items; let i = index; trackBy: trackByFn">...</li>
<template ngFor let-item [ngForOf]="items" let-i="index" [ngForTrackBy]="trackByFn">
See a live demo for a more detailed