List Rendering
Master list rendering in Aurelia with repeat.for. Learn efficient data binding, performance optimization, advanced patterns, and real-world techniques for dynamic collections including arrays, maps, s
The repeat.for binding is Aurelia's powerful list rendering mechanism that creates highly optimized, reactive displays of collection data. It intelligently tracks changes, minimizes DOM updates, and provides rich contextual information for sophisticated data presentation.
Core Concepts
The repeat.for Binding
repeat.for Bindingrepeat.for creates a template instance for each item in a collection, similar to a for...of loop but with intelligent DOM management:
<ul>
<li repeat.for="item of items">
${item.name}
</li>
</ul>JavaScript Analogy:
for (let item of items) {
// Aurelia creates DOM element for each item
console.log(item.name);
}Change Detection and Updates
Aurelia automatically observes collection changes and updates the DOM efficiently:
export class MyComponent {
items = [{ name: 'John' }, { name: 'Jane' }];
addItem() {
// Aurelia detects this change and updates DOM
this.items.push({ name: 'Bob' });
}
updateFirst() {
// This change is also detected
this.items[0] = { name: 'Johnny' };
}
}Important: Use array mutating methods (push, pop, splice, reverse, sort) for automatic detection. Direct index assignment works but requires the array reference to change for detection.
Performance Optimization with Keys
Why Keys Matter
Without keys, Aurelia recreates DOM elements when collections change. With keys, it reuses existing elements:
<!-- Without keys: recreates all DOM on reorder -->
<div repeat.for="user of users">
<input value.bind="user.name">
</div>
<!-- With keys: preserves DOM and form state -->
<div repeat.for="user of users; key.bind: user.id">
<input value.bind="user.name">
</div>Key Strategies
Property-based keys (recommended):
<!-- Use stable, unique properties -->
<li repeat.for="product of products; key.bind: product.id">
${product.name}
</li>Literal property keys (more efficient):
<!-- Avoids expression evaluation -->
<li repeat.for="product of products; key: id">
${product.name}
</li>Expression-based keys (flexible but slower):
<!-- For complex key logic -->
<li repeat.for="item of items; key.bind: item.category + '-' + item.id">
${item.name}
</li>When to Use Keys
Dynamic collections where items are added, removed, or reordered
Form inputs to preserve user input during updates
Stateful components to maintain component state
Large lists for performance optimization
Sortable/filterable lists
Avoid keys when:
Collection is static or append-only
Items are simple primitives without DOM state
Performance testing shows no benefit
Contextual Properties
Every repeat iteration provides rich contextual information:
<div repeat.for="item of items">
<span class="index">Item ${$index + 1} of ${$length}</span>
<span class="status">
${$first ? 'First' : $last ? 'Last' : $middle ? 'Middle' : ''}
</span>
<div class="item ${$even ? 'even' : 'odd'}">
${item.name}
</div>
</div>Complete Property Reference
$index
number
Zero-based index (0, 1, 2...)
$first
boolean
true for the first item
$last
boolean
true for the last item
$middle
boolean
true for items that aren't first or last
$even
boolean
true for even indices (0, 2, 4...)
$odd
boolean
true for odd indices (1, 3, 5...)
$length
number
Total number of items
$previous
any
null
$parent
object
Parent binding context
Nested Repeats and $parent
Access parent contexts in nested structures:
<div repeat.for="department of departments">
<h2>${department.name}</h2>
<div repeat.for="employee of department.employees">
<span>
Dept: ${$parent.department.name},
Employee #${$index + 1}: ${employee.name}
</span>
<!-- Access root context -->
<span>Company: ${$parent.$parent.companyName}</span>
</div>
</div>Accessing Previous Items with $previous
The $previous contextual property provides access to the previous iteration's item, enabling powerful comparison and rendering patterns. It is a computed property available by default as part of repeat's contextual values. You can disable all contextual computed values (including $previous) using the contextual option.
Basic usage:
<!-- $previous is enabled by default (disable with contextual: false) -->
<div repeat.for="item of items">
<div class="item">
${item.name}
<span if.bind="$previous !== null">
(Previous: ${$previous.name})
</span>
</div>
</div>Key characteristics:
$previousisnullfor the first item$previousisundefinedwhencontextualis disabledComputed property with minimal overhead when enabled (contextual is enabled by default)
Works with all collection types (arrays, Maps, Sets, etc.)
Compatible with keyed repeats
Section Headers and Dividers
A common use case is rendering section headers only when data changes:
export class ProductList {
products = [
{ category: 'Electronics', name: 'Laptop' },
{ category: 'Electronics', name: 'Mouse' },
{ category: 'Books', name: 'JavaScript Guide' },
{ category: 'Books', name: 'TypeScript Handbook' }
];
}<!-- Show category header only when it changes -->
<div repeat.for="product of products">
<h2 if.bind="product.category !== $previous?.category">
${product.category}
</h2>
<div class="product">${product.name}</div>
</div>Output:
Electronics
Laptop
Mouse
Books
JavaScript Guide
TypeScript HandbookComparison and Change Indicators
Highlight changes from previous values:
export class StockTracker {
prices = [
{ time: '09:00', price: 100 },
{ time: '09:01', price: 102 },
{ time: '09:02', price: 98 },
{ time: '09:03', price: 98 }
];
}<table>
<tr repeat.for="entry of prices">
<td>${entry.time}</td>
<td class="${entry.price > $previous?.price ? 'up' :
entry.price < $previous?.price ? 'down' : ''}">
$${entry.price}
<span if.bind="$previous && entry.price !== $previous.price">
${entry.price > $previous.price ? '↑' : '↓'}
</span>
</td>
</tr>
</table>Combining with Keys
$previous works seamlessly with keyed repeats:
<!-- Multiple iterator properties separated by semicolons -->
<div repeat.for="item of items; key: id">
<div class="item-${item.id}">
${item.name}
<span if.bind="$previous">
Changed from: ${$previous.name}
</span>
</div>
</div>Conditional Contextual Properties
Control contextual computed properties (including $previous) based on view model properties:
export class ConfigurableList {
items = [...];
showContextual = true; // Toggle contextual on/off
}<!-- Enable/disable contextual based on component state -->
<div repeat.for="item of items; contextual.bind: showContextual">
<!-- $previous is only available when contextual is true -->
</div>Performance Considerations
When contextual is disabled:
Zero memory overhead -
$previousis not computedNegligible CPU cost - single conditional check per item
When contextual is enabled (default):
Computed on demand via contextual getter
Minimal CPU cost
Best practices:
Keep contextual enabled unless you have a strong reason to disable it
If needed, disable per-instance with
contextual: falseorcontextual.bind: someBoolean
Data Types and Collections
Arrays
The most common and optimized collection type:
export class ProductList {
products = [
{ id: 1, name: 'Laptop', price: 999 },
{ id: 2, name: 'Mouse', price: 25 }
];
sortByPrice() {
// Aurelia detects and updates DOM
this.products.sort((a, b) => a.price - b.price);
}
}<div repeat.for="product of products; key.bind: product.id">
<h3>${product.name}</h3>
<span class="price">${product.price | currency}</span>
</div>Sets
Useful for unique collections:
export class TagManager {
selectedTags = new Set(['javascript', 'typescript']);
toggleTag(tag: string) {
if (this.selectedTags.has(tag)) {
this.selectedTags.delete(tag);
} else {
this.selectedTags.add(tag);
}
}
}<div repeat.for="tag of selectedTags">
<span class="tag">${tag}</span>
</div>Maps
Perfect for key-value pairs:
export class LocalizationDemo {
translations = new Map([
['en', 'Hello'],
['es', 'Hola'],
['fr', 'Bonjour']
]);
}<!-- Destructure map entries -->
<div repeat.for="[language, greeting] of translations">
<strong>${language}:</strong> ${greeting}
</div>
<!-- Or access as entry object -->
<div repeat.for="entry of translations">
<strong>${entry[0]}:</strong> ${entry[1]}
</div>Number Ranges
Generate sequences quickly:
<!-- Create pagination -->
<nav>
<a repeat.for="page of totalPages"
href="/products?page=${page + 1}">
${page + 1}
</a>
</nav>
<!-- Star ratings -->
<div class="rating">
<span repeat.for="star of 5"
class="star ${star < rating ? 'filled' : ''}">
★
</span>
</div>Advanced Patterns
Destructuring Declarations
Extract multiple values in the repeat declaration:
export class OrderHistory {
orders = [
{ id: 1, items: [{ name: 'Coffee', qty: 2 }] },
{ id: 2, items: [{ name: 'Tea', qty: 1 }] }
];
}<!-- Destructure objects -->
<div repeat.for="{ id, items } of orders">
Order #${id}: ${items.length} items
</div>
<!-- Destructure arrays -->
<div repeat.for="[index, value] of arrayOfPairs">
${index}: ${value}
</div>Integration with Other Template Controllers
Conditional rendering within repeats:
<div repeat.for="user of users">
<div if.bind="user.isActive">
<strong>${user.name}</strong> - Active
</div>
<div else>
<em>${user.name}</em> - Inactive
</div>
</div>Nested conditionals and repeats:
<div repeat.for="category of categories">
<h2>${category.name}</h2>
<div if.bind="category.products.length > 0">
<div repeat.for="product of category.products; key.bind: product.id">
${product.name}
</div>
</div>
<p else>No products in this category</p>
</div>Working with Async Data
Handle loading states and async operations:
export class AsyncDataExample {
items: Item[] = [];
isLoading = true;
error: string | null = null;
async attached() {
try {
this.items = await this.dataService.getItems();
} catch (err) {
this.error = err.message;
} finally {
this.isLoading = false;
}
}
}<div if.bind="isLoading">
<spinner></spinner> Loading...
</div>
<div else>
<div if.bind="error">
<div class="error">Error: ${error}</div>
</div>
<div else>
<div if.bind="items.length === 0">
<p>No items found</p>
</div>
<div else>
<div repeat.for="item of items; key.bind: item.id">
${item.name}
</div>
</div>
</div>
</div>Complex Object Iteration
Use value converters for non-standard collections:
// Object keys converter
export class KeysValueConverter {
toView(obj: Record<string, any>): string[] {
return obj ? Object.keys(obj) : [];
}
}
// Object entries converter
export class EntriesValueConverter {
toView(obj: Record<string, any>): [string, any][] {
return obj ? Object.entries(obj) : [];
}
}<!-- Iterate object keys -->
<div repeat.for="key of settings | keys">
<label>${key}:</label>
<input value.bind="settings[key]">
</div>
<!-- Iterate object entries -->
<div repeat.for="[key, value] of configuration | entries">
<strong>${key}:</strong> ${value}
</div>Performance Best Practices
Optimizing Large Lists
Use keyed iteration:
<!-- Enables efficient DOM reuse -->
<div repeat.for="item of largeList; key.bind: item.id">
${item.name}
</div>Consider virtual scrolling for very large lists:
<!-- Use ui-virtualization for very large collecitons of items -->
<div virtual-repeat.for="item of hugeList">
${item.name}
</div>This requires using the virtual repeat plugin.
Memory Management
Avoid memory leaks in complex scenarios:
export class ListComponent {
private subscription?: IDisposable;
attached() {
// Subscribe to external data changes
this.subscription = this.dataService.changes.subscribe(
items => this.items = items
);
}
detaching() {
// Clean up subscriptions
this.subscription?.dispose();
}
}Custom Collection Handlers
Built-in Handlers
Aurelia includes handlers for:
Arrays (
Array,[])Sets (
Set)Maps (
Map)Numbers (
5→ creates range 0-4)Array-like objects (NodeList, HTMLCollection, etc.)
Null/undefined (renders nothing)
Creating Custom Handlers
For specialized collections:
import { IRepeatableHandler, Registration } from 'aurelia';
// Custom handler for immutable lists
class ImmutableListHandler implements IRepeatableHandler {
handles(value: unknown): boolean {
return value && typeof value === 'object' && 'size' in value && 'get' in value;
}
iterate(value: any, func: (item: unknown, index: number) => void): void {
for (let i = 0; i < value.size; i++) {
func(value.get(i), i);
}
}
}
// Register the handler
Aurelia.register(
Registration.singleton(IRepeatableHandler, ImmutableListHandler)
).app(MyApp).start();Observable Collections
Create reactive custom collections:
import { CollectionObserver, ICollectionObserver } from '@aurelia/runtime';
class ReactiveCustomCollection {
private _items: any[] = [];
private _observer?: ICollectionObserver;
get items() { return this._items; }
add(item: any) {
this._items.push(item);
this._observer?.handleCollectionChange(/* change details */);
}
// Implement observable pattern...
}Troubleshooting Common Issues
Issue: Changes Not Reflecting
Problem: Direct array index assignment doesn't trigger updates
// This won't update the DOM
this.items[0] = newItem;Solution: Use array methods or replace the array
// These will update the DOM
this.items.splice(0, 1, newItem);
// or
this.items = [...this.items.slice(0, 0), newItem, ...this.items.slice(1)];Issue: Form State Lost on Reorder
Problem: Input values disappear when list is reordered
<!-- No keys = DOM recreation -->
<div repeat.for="item of items">
<input value.bind="item.name">
</div>Solution: Use stable keys
<!-- Keys preserve DOM elements -->
<div repeat.for="item of items; key.bind: item.id">
<input value.bind="item.name">
</div>Issue: Performance with Large Lists
Problem: Slow rendering with 1000+ items
Solutions:
Use virtual scrolling for very large lists
Implement pagination or infinite scroll
Optimize templates - minimize complex expressions
Use keys to enable DOM reuse
Issue: Memory Leaks
Problem: Components not disposing properly
Solution: Clean up in lifecycle hooks
export class MyComponent {
detaching() {
// Dispose of subscriptions, timers, etc.
this.cleanup();
}
}Real-World Examples
Dynamic Product Catalog
export class ProductCatalog {
products: Product[] = [];
filteredProducts: Product[] = [];
searchTerm = '';
selectedCategory = '';
searchTermChanged() {
this.filterProducts();
}
categoryChanged() {
this.filterProducts();
}
private filterProducts() {
this.filteredProducts = this.products.filter(product => {
const matchesSearch = !this.searchTerm ||
product.name.toLowerCase().includes(this.searchTerm.toLowerCase());
const matchesCategory = !this.selectedCategory ||
product.category === this.selectedCategory;
return matchesSearch && matchesCategory;
});
}
}<div class="filters">
<input value.bind="searchTerm" placeholder="Search products...">
<select value.bind="selectedCategory">
<option value="">All Categories</option>
<option repeat.for="category of categories"
value.bind="category">${category}</option>
</select>
</div>
<div class="product-grid">
<div repeat.for="product of filteredProducts; key.bind: product.id"
class="product-card">
<img src.bind="product.image" alt.bind="product.name">
<h3>${product.name}</h3>
<p class="price">${product.price | currency}</p>
<button click.trigger="addToCart(product)">Add to Cart</button>
</div>
</div>
<div if.bind="filteredProducts.length === 0" class="no-results">
No products found matching your criteria.
</div>Data Table with Sorting
export class DataTable {
data: TableRow[] = [];
sortColumn = '';
sortDirection: 'asc' | 'desc' = 'asc';
sort(column: string) {
if (this.sortColumn === column) {
this.sortDirection = this.sortDirection === 'asc' ? 'desc' : 'asc';
} else {
this.sortColumn = column;
this.sortDirection = 'asc';
}
this.data.sort((a, b) => {
const aVal = a[column];
const bVal = b[column];
const modifier = this.sortDirection === 'asc' ? 1 : -1;
return aVal < bVal ? -modifier : aVal > bVal ? modifier : 0;
});
}
}<table class="data-table">
<thead>
<tr>
<th repeat.for="column of columns"
click.trigger="sort(column.key)"
class="${sortColumn === column.key ? 'sorted ' + sortDirection : ''}">
${column.title}
<span if.bind="sortColumn === column.key"
class="sort-indicator">
${sortDirection === 'asc' ? '↑' : '↓'}
</span>
</th>
</tr>
</thead>
<tbody>
<tr repeat.for="row of data; key.bind: row.id">
<td repeat.for="column of columns">
${row[column.key] | column.converter}
</td>
</tr>
</tbody>
</table>TypeScript Integration
Type-Safe Repeats
interface User {
id: number;
name: string;
email: string;
isActive: boolean;
}
export class UserList {
users: User[] = [];
// Type-safe filtering
get activeUsers(): User[] {
return this.users.filter(user => user.isActive);
}
// Type-safe operations
toggleUserStatus(user: User): void {
user.isActive = !user.isActive;
}
}<!-- TypeScript provides intellisense and type checking -->
<div repeat.for="user of activeUsers; key.bind: user.id">
<span>${user.name}</span> <!-- ✓ TypeScript knows user.name exists -->
<span>${user.email}</span> <!-- ✓ Type safe -->
<button click.trigger="toggleUserStatus(user)">
${user.isActive ? 'Deactivate' : 'Activate'}
</button>
</div>Last updated
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