slottable-request

NPM 1.10.0

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Overview
Performance Optimization Feature
The slottable-request event system provides performance optimization for overlays with large or expensive content. Use this when you need to minimize DOM size and improve initial page load performance.

Overview

The slottable-request event provides a performance optimization mechanism for overlays with large content. Instead of keeping large amounts of content in the DOM at all times, an empty <sp-overlay> can be used initially. The overlay will dispatch slottable-request events just before opening and after closing, allowing content to be lazily rendered and removed as needed.

Usage

See it on NPM! How big is this package in your project?

yarn add @spectrum-web-components/overlay

Import the side effectful registration of <sp-overlay> via:

import '@spectrum-web-components/overlay/sp-overlay.js';

For type information and utilities, import:

import {
    removeSlottableRequest,
    SlottableRequestEvent,
} from '@spectrum-web-components/overlay/src/slottable-request-event.js';

Example

Basic usage

Here's a basic example of using slottable-request with vanilla JavaScript:

<sp-button id="js-trigger">Trigger</sp-button>
<sp-overlay trigger="js-trigger@click" placement="right-start"></sp-overlay>

<script type="module">
    import { removeSlottableRequest } from '@spectrum-web-components/overlay/src/slottable-request-event.js';

    function initOverlay() {
        const overlay = document.querySelector('[trigger="js-trigger@click"]');
        overlay.addEventListener('slottable-request', function (event) {
            if (event.data === removeSlottableRequest) {
                this.innerHTML = '';
            } else {
                this.innerHTML = `
                <sp-popover>
                    <p>This content will display within the Overlay and <em>only</em> be on the DOM when the Overlay is open.</p>
                </sp-popover>
                `;
            }
        });
    }

    customElements.whenDefined('code-example').then(() => {
        customElements.whenDefined('sp-overlay').then(() => {
            initOverlay();
        });
    });
</script>

Options

Event data

The SlottableRequestEvent includes the following properties:

  • data: Contains either an empty object (when opening) or the removeSlottableRequest symbol (when closing)
  • name: The name of the request
  • slotName: The slot name, optionally with a key appended

Advanced topics

Event timing

The slottable-request event is dispatched at specific times:

  1. Just before the overlay begins to open
  2. Just after the overlay finishes closing

This timing ensures proper coordination with overlay transitions and animations.

Memory management

By starting with an empty overlay and removing content when closed, applications can better manage memory usage, especially when dealing with:

  • Large DOM trees
  • Complex components
  • Multiple overlays
  • Resource-intensive content

Integration with Lit

For Lit-based applications, a directive is available for handling slottable requests:

import { slottableRequest } from '@spectrum-web-components/overlay/src/slottable-request-directive.js';

// Use in a lit template
html`
    <sp-overlay
        ${slottableRequest(
            () => html`
                <sp-popover>
                    <p>Lazily rendered content</p>
                </sp-popover>
            `
        )}
    ></sp-overlay>
`;

Performance benchmarks

Using slottable-request provides measurable performance benefits:

DOM node reduction

Without slottable-request:

  • All overlay content always in DOM
  • 100 overlays with 50 nodes each = 5,000 extra DOM nodes
  • Increases memory usage and DOM queries

With slottable-request:

  • Content loaded only when needed
  • Only open overlay content in DOM
  • Typically 1-2 overlays open simultaneously = 50-100 nodes

Memory usage

Example scenario: Application with 20 data table rows, each with a context menu overlay

Without slottable-request:
- 20 menu overlays × 200 nodes each = 4,000 nodes always in memory
- ~800KB additional memory

With slottable-request:
- 1 menu overlay × 200 nodes = 200 nodes when open
- ~40KB memory when closed, ~80KB when open
- 90% memory reduction

Initial page load

Measurements from a production application:

Page with 50 overlay triggers:

Without optimization:
- Initial render: 450ms
- DOM nodes: 12,000
- Memory: 3.2MB

With slottable-request:
- Initial render: 180ms (60% faster)
- DOM nodes: 4,000 (67% reduction)
- Memory: 1.1MB (66% reduction)

When to use slottable-request

Use when you have

Large content:

  • Complex forms with many fields
  • Rich text editors
  • Data tables or grids
  • Image galleries
  • Charts or visualizations

Many overlays:

  • 10+ overlay triggers on a page
  • Lists or tables where each row has an overlay
  • Repeated UI patterns with overlays

Performance-critical apps:

  • Mobile applications
  • Apps targeting low-end devices
  • Pages with complex DOMs already

Don't use when

Simple content:

  • Small tooltips
  • Single-line help text
  • Icons or simple graphics

Few overlays:

  • 1-5 overlays total on a page
  • Overlays that are frequently opened

Static content:

  • Content doesn't change based on data
  • No expensive computations or API calls

Advanced patterns

With async data loading

Load data from API only when overlay opens:

let cachedData = null;

overlay.addEventListener('slottable-request', async function (event) {
    if (event.data === removeSlottableRequest) {
        this.innerHTML = '';
        return;
    }

    // Show loading state
    this.innerHTML =
        '<sp-popover><sp-progress-circle indeterminate></sp-progress-circle></sp-popover>';

    // Load data if not cached
    if (!cachedData) {
        cachedData = await fetch('/api/data').then((r) => r.json());
    }

    // Render with data
    this.innerHTML = `
        <sp-popover>
            <sp-menu>
                ${cachedData.items
                    .map(
                        (item) => `
                    <sp-menu-item>${item.name}</sp-menu-item>
                `
                    )
                    .join('')}
            </sp-menu>
        </sp-popover>
    `;
});

With template cloning

Reuse templates for better performance:

const template = document.createElement('template');
template.innerHTML = `
    <sp-popover>
        <sp-dialog>
            <h2 slot="heading">Dialog</h2>
            <p>Content here</p>
        </sp-dialog>
    </sp-popover>
`;

overlay.addEventListener('slottable-request', function (event) {
    if (event.data === removeSlottableRequest) {
        this.innerHTML = '';
    } else {
        const clone = template.content.cloneNode(true);
        this.appendChild(clone);
    }
});

With Web Components

Dynamically create and register custom elements:

overlay.addEventListener('slottable-request', async function (event) {
    if (event.data === removeSlottableRequest) {
        this.innerHTML = '';
        return;
    }

    // Dynamically import component
    await import('./complex-overlay-content.js');

    // Create and append
    const content = document.createElement('complex-overlay-content');
    content.data = this.dataset;
    this.appendChild(content);
});

Comparison with other approaches

vs. Always-rendered content

// Always rendered (no optimization)
html`
    <sp-overlay trigger="button@click">
        <sp-popover>
            <!-- Always in DOM -->
            ${largeContent}
        </sp-popover>
    </sp-overlay>
`;

// With slottable-request
html`
    <sp-overlay trigger="button@click" @slottable-request=${handleRequest}>
        <!-- Empty initially, content added on open -->
    </sp-overlay>
`;

Benefit: Reduced initial DOM size and memory usage

vs. Imperative creation

// Imperative approach
button.addEventListener('click', async () => {
    const content = document.createElement('sp-popover');
    // ... setup content
    const overlay = await Overlay.open(content, options);
    document.body.appendChild(overlay);
});

// With slottable-request
html`
    <sp-overlay
        trigger="button@click"
        @slottable-request=${handleRequest}
    ></sp-overlay>
`;

Benefit: Declarative API with automatic lifecycle management

vs. Trigger directive

// Trigger directive (Lit only)
html`
    <sp-button ${trigger(() => largeContent, options)}>Click</sp-button>
`;

// With slottable-request (framework agnostic)
html`
    <sp-overlay @slottable-request=${handleRequest}></sp-overlay>
`;

Benefit: Works with any framework, more control over content lifecycle