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 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

Property	Type	Description
$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:

• $previous is null for the first item

• $previous is undefined when contextual is disabled

• Computed 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 Handbook

Comparison 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 - $previous is not computed

• Negligible 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: false or contextual.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:

1. Use virtual scrolling for very large lists

2. Implement pagination or infinite scroll

3. Optimize templates - minimize complex expressions

4. 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>