List Rendering

Learn how to work with collections of data like arrays and maps. The repeat.for functionality allows you to loop over collections of data and work with their contents similar to a Javascript for loop.

Aurelia’s templating system offers a robust way to work with collections—be they arrays, sets, maps, or even ranges. The repeat.for binding provides a declarative approach to iterating over data, creating scopes for each iteration, and optimizing DOM updates. This guide explains the intricacies of list rendering in detail.

The `repeat.for` Binding

At its core, repeat.for acts like a template-based for...of loop. It iterates over a collection and creates a new rendering context for each item. For example:

<ul>
  <li repeat.for="item of items">
    ${item.name}
  </li>
</ul>

This snippet tells Aurelia to:

Loop over each element in the items collection.
Assign the current element to a local variable named item.
Render the element (here, displaying item.name) for each iteration.

JavaScript Analogy:

for (let item of items) {
  console.log(item.name);
}

Keyed Iteration for Efficient DOM Updates

When working with dynamic collections, it’s crucial to minimize unnecessary DOM operations. Aurelia allows you to specify a unique key for each item in the collection. This key helps the framework:

Track Changes: By comparing key values, Aurelia can identify which items have been added, removed, or reordered.
Optimize Updates: Only the modified elements are updated, preserving performance.
Maintain State: DOM elements (e.g., with user input or focus) retain their state even if their order changes.

Syntax Examples

You can declare the key using either literal or binding syntax:

<!-- Literal syntax -->
<ul>
  <li repeat.for="item of items; key: id">
    ${item.name}
  </li>
</ul>

<!-- Bound syntax -->
<ul>
  <li repeat.for="item of items; key.bind: item.id">
    ${item.name}
  </li>
</ul>

Guidelines for Keys:

Uniqueness: Use a property that uniquely identifies each item (like an id).
Stability: Avoid using array indices if the collection order can change.

Contextual Properties in Repeats

Inside a repeat.for block, Aurelia exposes several contextual properties that give you more control over the rendering logic:

$index: The zero-based index of the current iteration.
$first: A boolean that is true on the first iteration.
$last: A boolean that is true on the final iteration.
$even / $odd: Flags indicating whether the current index is even or odd, which is useful for styling alternating rows.
$length: The total number of items in the collection.
$parent: A reference to the parent binding context. This is especially useful in nested repeaters.

Example Usage

<ul>
  <li repeat.for="item of items">
    Index: ${$index} — Name: ${item.name} <br>
    Is first? ${$first} | Is last? ${$last} <br>
    Even? ${$even} | Odd? ${$odd} <br>
    Total items: ${$length}
  </li>
</ul>

For nested repeats, you can access the outer scope with $parent:

<ul>
  <li repeat.for="item of items">
    Outer index: ${$parent.$index}
  </li>
</ul>

Working with Arrays

Arrays are the most common data source for repeats. Here’s an example component and its template:

Component (my-component.ts):

export class MyComponent {
  items = [
    { name: 'John' },
    { name: 'Bill' }
  ];
}

Template (my-component.html):

<ul>
  <li repeat.for="item of items">
    ${item.name}
  </li>
</ul>

Note: Aurelia tracks changes in arrays when you use array methods like push, pop, or splice. Direct assignments (e.g., array[index] = value) won’t trigger change detection.

Generating Ranges

repeat.for isn’t limited to collections—it can also generate a sequence of numbers. For instance, to create a countdown:

<p repeat.for="i of 10">
  ${10 - i}
</p>
<p>Blast Off!</p>

This iterates 10 times and computes 10 - i on each pass.

Iterating Sets

Sets are handled much like arrays. The syntax remains the same, though the underlying collection is a Set.

Component (repeater-template.ts):

export class RepeaterTemplate {
  friends: Set<string> = new Set(['Alice', 'Bob', 'Carol', 'Dana']);
}

Template (repeater-template.html):

<template>
  <p repeat.for="friend of friends">
    Hello, ${friend}!
  </p>
</template>

Iterating Maps

Maps offer a powerful way to iterate key-value pairs. Aurelia lets you deconstruct the map entry directly in the template.

Component (repeater-template.ts):

export class RepeaterTemplate {
  friends = new Map([
    ['Hello', { name: 'Alice' }],
    ['Hola', { name: 'Bob' }],
    ['Ni Hao', { name: 'Carol' }],
    ['Molo', { name: 'Dana' }]
  ]);
}

Template (repeater-template.html):

<p repeat.for="[greeting, friend] of friends">
  ${greeting}, ${friend.name}!
</p>

Here, [greeting, friend] splits each map entry so you can access both the key (greeting) and value (friend).

Iterating Objects via Value Converters

Objects aren’t directly iterable in Aurelia. To iterate over an object’s properties, convert it into an iterable format using a value converter.

Creating a Keys Value Converter

// resources/value-converters/keys.ts
export class KeysValueConverter {
  toView(obj: object): string[] {
    return Reflect.ownKeys(obj) as string[];
  }
}

Using the Converter in a Template

<import from="resources/value-converters/keys"></import>

<p repeat.for="key of friends | keys">
  ${key}, ${friends[key].name}!
</p>

The keys converter transforms the object into an array of its keys, making it iterable by repeat.for.

Custom Collection Handling with Repeat Handlers

Aurelia’s repeat system is extensible. If you need to iterate over non-standard collections (like HTMLCollections, NodeLists, or FileLists), you can create a custom repeat handler by implementing the IRepeatableHandler interface.

Custom Handler Example

import Aurelia, { Registration, IRepeatableHandler } from 'aurelia';

class ArrayLikeHandler implements IRepeatableHandler {
  static register(container) {
    Registration.singleton(IRepeatableHandler, ArrayLikeHandler).register(container);
  }

  handles(value: any): boolean {
    return 'length' in value && typeof value.length === 'number';
  }

  iterate(items: any, callback: (item: any, index: number) => void): void {
    for (let i = 0, len = items.length; i < len; i++) {
      callback(items[i], i);
    }
  }
}

Aurelia.register(
  ArrayLikeHandler,
  // other registrations
).app(MyApp).start();

Tip: Aurelia provides a default ArrayLikeHandler you can import directly:
import Aurelia, { ArrayLikeHandler } from 'aurelia';

Custom Handler Resolver

If you need to override the default order in which Aurelia selects a repeat handler, you can implement your own IRepeatableHandlerResolver:

class MyRepeatableHandlerResolver {
  resolve(value: any) {
    if (typeof value?.length === 'number') {
      return {
        iterate(items, callback) {
          for (let i = 0; i < items.length; ++i) {
            callback(items[i], i, items);
          }
        }
      };
    }
    throw new Error('The repeater supports only array-like objects.');
  }
}

This custom resolver can redefine how different collection types are handled by the repeater.

Summary

Aurelia 2’s list rendering capabilities are both powerful and flexible:

Versatile Iteration: Work with arrays, sets, maps, ranges, and even objects (via converters).
Efficient Updates: Use keyed iteration to minimize DOM changes.
Contextual Data: Access properties like $index, $first, $last, $even, $odd, $length, and $parent for richer templates.
Extensibility: Create custom handlers and resolvers to support any iterable data structure.

Mastering these features enables you to build dynamic, efficient UIs that handle complex data sets with ease.

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The repeat.for Binding