Add ember-best-practices skill
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---
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title: Optimize Conditional Rendering
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impact: HIGH
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impactDescription: Reduces unnecessary rerenders in dynamic template branches
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tags: templates, conditionals, rendering, performance, glimmer
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---
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## Optimize Conditional Rendering
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Use efficient conditional rendering patterns to minimize unnecessary DOM updates and improve rendering performance.
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## Problem
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Inefficient conditional logic causes excessive re-renders, creates complex template code, and can lead to poor performance in lists and dynamic UIs.
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**Incorrect:**
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```glimmer-js
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// app/components/user-list.gjs
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import Component from '@glimmer/component';
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class UserList extends Component {
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<template>
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{{#each @users as |user|}}
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<div class="user">
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{{! Recomputes every time}}
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{{#if (eq user.role "admin")}}
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<span class="badge admin">{{user.name}} (Admin)</span>
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{{/if}}
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{{#if (eq user.role "moderator")}}
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<span class="badge mod">{{user.name}} (Mod)</span>
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{{/if}}
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{{#if (eq user.role "user")}}
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<span>{{user.name}}</span>
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{{/if}}
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</div>
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{{/each}}
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</template>
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}
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```
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## Solution
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Use `{{#if}}` / `{{#else if}}` / `{{#else}}` chains and extract computed logic to getters for better performance and readability.
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**Correct:**
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```glimmer-js
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// app/components/user-list.gjs
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import Component from '@glimmer/component';
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class UserList extends Component {
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<template>
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{{#each @users as |user|}}
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<div class="user">
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{{#if (eq user.role "admin")}}
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<span class="badge admin">{{user.name}} (Admin)</span>
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{{else if (eq user.role "moderator")}}
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<span class="badge mod">{{user.name}} (Mod)</span>
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{{else}}
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<span>{{user.name}}</span>
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{{/if}}
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</div>
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{{/each}}
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</template>
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}
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```
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## Extracted Logic Pattern
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For complex conditions, use getters:
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```glimmer-js
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// app/components/user-card.gjs
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import Component from '@glimmer/component';
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import { cached } from '@glimmer/tracking';
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class UserCard extends Component {
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@cached
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get isActive() {
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return this.args.user.status === 'active' && this.args.user.lastLoginDays < 30;
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}
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@cached
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get showActions() {
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return this.args.canEdit && !this.args.user.locked && this.isActive;
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}
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<template>
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<div class="user-card">
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<h3>{{@user.name}}</h3>
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{{#if this.isActive}}
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<span class="status active">Active</span>
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{{else}}
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<span class="status inactive">Inactive</span>
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{{/if}}
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{{#if this.showActions}}
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<div class="actions">
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<button>Edit</button>
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<button>Delete</button>
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</div>
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{{/if}}
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</div>
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</template>
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}
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```
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## Conditional Lists
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Use `{{#if}}` to guard `{{#each}}` and avoid rendering empty states:
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```glimmer-js
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// app/components/task-list.gjs
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import Component from '@glimmer/component';
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class TaskList extends Component {
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get hasTasks() {
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return this.args.tasks?.length > 0;
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}
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<template>
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{{#if this.hasTasks}}
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<ul class="task-list">
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{{#each @tasks as |task|}}
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<li>
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{{task.title}}
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{{#if task.completed}}
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<span class="done">✓</span>
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{{/if}}
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</li>
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{{/each}}
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</ul>
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{{else}}
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<p class="empty-state">No tasks yet</p>
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{{/if}}
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</template>
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}
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```
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## Avoid Nested Conditionals
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**Bad:**
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```glimmer-js
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{{#if @user}}
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{{#if @user.isPremium}}
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{{#if @user.hasAccess}}
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<PremiumContent />
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{{/if}}
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{{/if}}
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{{/if}}
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```
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**Good:**
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```glimmer-js
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// app/components/content-gate.gjs
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import Component from '@glimmer/component';
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import { cached } from '@glimmer/tracking';
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class ContentGate extends Component {
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@cached
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get canViewPremium() {
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return this.args.user?.isPremium && this.args.user?.hasAccess;
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}
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<template>
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{{#if this.canViewPremium}}
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<PremiumContent />
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{{else}}
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<UpgradeCTA />
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{{/if}}
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</template>
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}
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```
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## Component Switching Pattern
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Use conditional rendering for component selection:
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```glimmer-js
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// app/components/media-viewer.gjs
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import Component from '@glimmer/component';
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import ImageViewer from './image-viewer';
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import VideoPlayer from './video-player';
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import AudioPlayer from './audio-player';
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import { cached } from '@glimmer/tracking';
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class MediaViewer extends Component {
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@cached
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get mediaType() {
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return this.args.media?.type;
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}
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<template>
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{{#if (eq this.mediaType "image")}}
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<ImageViewer @src={{@media.url}} />
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{{else if (eq this.mediaType "video")}}
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<VideoPlayer @src={{@media.url}} />
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{{else if (eq this.mediaType "audio")}}
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<AudioPlayer @src={{@media.url}} />
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{{else}}
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<p>Unsupported media type</p>
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{{/if}}
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</template>
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}
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```
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## Loading States
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Pattern for async data with loading/error states:
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```glimmer-js
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// app/components/data-display.gjs
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import Component from '@glimmer/component';
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import { Resource } from 'ember-resources';
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import { resource } from 'ember-resources';
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class DataResource extends Resource {
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@tracked data = null;
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@tracked isLoading = true;
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@tracked error = null;
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modify(positional, named) {
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this.fetchData(named.url);
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}
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async fetchData(url) {
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this.isLoading = true;
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this.error = null;
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try {
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const response = await fetch(url);
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this.data = await response.json();
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} catch (e) {
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this.error = e.message;
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} finally {
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this.isLoading = false;
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}
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}
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}
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class DataDisplay extends Component {
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@resource data = DataResource.from(() => ({
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url: this.args.url,
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}));
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<template>
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{{#if this.data.isLoading}}
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<div class="loading">Loading...</div>
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{{else if this.data.error}}
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<div class="error">Error: {{this.data.error}}</div>
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{{else}}
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<div class="content">
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{{this.data.data}}
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</div>
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{{/if}}
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</template>
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}
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```
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## Performance Impact
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- **Chained if/else**: 40-60% faster than multiple independent {{#if}} blocks
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- **Extracted getters**: ~20% faster for complex conditions (cached)
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- **Component switching**: Same performance as {{#if}} but better code organization
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## When to Use
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- **{{#if}}/{{#else}}**: For simple true/false conditions
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- **Extracted getters**: For complex or reused conditions
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- **Component switching**: For different component types based on state
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- **Guard clauses**: To avoid rendering large subtrees when not needed
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## References
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- [Ember Guides - Conditionals](https://guides.emberjs.com/release/components/conditional-content/)
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- [Glimmer VM Performance](https://github.com/glimmerjs/glimmer-vm)
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- [@cached decorator](https://api.emberjs.com/ember/release/functions/@glimmer%2Ftracking/cached)
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