Dynamic Panel Component Pattern

Create dashboard panel components using vanilla TypeScript (no framework, no JSX). Each panel is a class extending a Panel base class, with enhanced dynamic content updates and error handling.

Architecture Overview

Panel (base class)
├── element: HTMLElement (outer container, .panel)
│   ├── header: HTMLElement (.panel-header)
│   │   ├── headerLeft (.panel-header-left)
│   │   │   ├── title (.panel-title)
│   │   │   └── newBadge (.panel-new-badge) [optional]
│   │   ├── statusBadge (.panel-data-badge) [optional]
│   │   └── countEl (.panel-count) [optional]
│   ├── content: HTMLElement (.panel-content)
│   └── resizeHandle (.panel-resize-handle)

Base Panel Class (Enhanced)

Create src/components/Panel.ts:

export interface PanelOptions {
  id: string;
  title: string;
  showCount?: boolean;
  className?: string;
}

export class Panel {
  protected element: HTMLElement;
  protected content: HTMLElement;
  protected header: HTMLElement;
  protected countEl: HTMLElement | null = null;
  protected panelId: string;
  private _fetching = false;
  private _errorState = false;

  constructor(options: PanelOptions) {
    this.panelId = options.id;
    this.element = document.createElement('div');
    this.element.className = `panel ${options.className || ''}`;
    this.element.dataset.panel = options.id;

    // Header
    this.header = document.createElement('div');
    this.header.className = 'panel-header';

    const headerLeft = document.createElement('div');
    headerLeft.className = 'panel-header-left';

    const title = document.createElement('span');
    title.className = 'panel-title';
    title.textContent = options.title;
    headerLeft.appendChild(title);
    this.header.appendChild(headerLeft);

    // Count badge (optional)
    if (options.showCount) {
      this.countEl = document.createElement('span');
      this.countEl.className = 'panel-count';
      this.countEl.textContent = '0';
      this.header.appendChild(this.countEl);
    }

    // Content area
    this.content = document.createElement('div');
    this.content.className = 'panel-content';
    this.content.id = `${options.id}Content`;

    this.element.appendChild(this.header);
    this.element.appendChild(this.content);
    this.showLoading();
  }

  public getElement(): HTMLElement { return this.element; }

  public showLoading(message = 'Loading...'): void {
    this._errorState = false;
    this.content.innerHTML = `
     <div class="panel-loading">
        <div class="panel-loading-spinner"></div>
        <div class="panel-loading-text">${message}</div>
      </div>`;
  }

  public showError(message = 'Failed to load', onRetry?: () => void): void {
    this._errorState = true;
    this.content.innerHTML = `
      <div class="panel-error-state">
        <div class="panel-error-msg">${message}</div>
        ${onRetry ? '<button class="panel-retry-btn" data-panel-retry>Retry</button>' : ''}
      </div>`;
    if (onRetry) {
      this.content.querySelector('[data-panel-retry]')?.addEventListener('click', onRetry);
    }
  }

  public setContent(html: string): void {
    this._errorState = false;
    this.content.innerHTML = html;
  }

  public setCount(count: number): void {
    if (this.countEl) this.countEl.textContent = count.toString();
  }

  public show(): void { this.element.classList.remove('hidden'); }
  public hide(): void { this.element.classList.add('hidden'); }

  protected setFetching(v: boolean): void { this._fetching = v; }
  protected get isFetching(): boolean { return this._fetching; }
  protected get isErrorState(): boolean { return this._errorState; }

  public destroy(): void {
    this.element.remove();
  }
}

Creating a Concrete Panel (Example: StockPanel with Enhanced Error Handling)

import { Panel } from './Panel';

interface StockQuote {
  symbol: string;
  name: string;
  price: number | null;
  change: number | null;
  sparkline?: number[];
}

export class StockPanel extends Panel {
  private refreshTimer: ReturnType<typeof setInterval> | null = null;
  private retryCount = 0;
  private maxRetries = 3;

  constructor() {
    super({ id: 'stocks', title: 'Stock Market', showCount: true });
    this.fetchData();
    this.refreshTimer = setInterval(() => this.fetchData(), 60_000);
  }

  private async fetchData(): Promise<void> {
    if (this.isFetching || this.isErrorState) return;
    this.setFetching(true);
    try {
      const quotes = await fetchStockQuotes(); // from data-service
      this.render(quotes);
      this.setCount(quotes.length);
      this.retryCount = 0; // Reset retry count on success
    } catch (err) {
      this.retryCount++;
      if (this.retryCount < this.maxRetries) {
        setTimeout(() => this.fetchData(), 1000 * this.retryCount); // Exponential backoff
      } else {
        this.showError('Failed to load stock data', () => {
          this.retryCount = 0;
          this.fetchData();
        });
      }
    } finally {
      this.setFetching(false);
    }
  }

  private render(quotes: StockQuote[]): void {
    const rows = quotes.map(q => `
      <div class="stock-row">
        <span class="stock-symbol">${q.symbol}</span>
        <span class="stock-name">${q.name}</span>
        <span class="stock-price">${q.price != null ? '$' + q.price.toFixed(2) : '—'}</span>
        <span class="stock-change ${(q.change ?? 0) >= 0 ? 'positive' : 'negative'}">
          ${q.change != null ? (q.change >= 0 ? '+' : '') + q.change.toFixed(2) + '%' : '—'}
        </span>
        ${miniSparkline(q.sparkline, q.change)}
      </div>
    `).join('');

    this.setContent(`<div class="stock-list">${rows}</div>`);
  }

  public override destroy(): void {
    if (this.refreshTimer) clearInterval(this.refreshTimer);
    super.destroy();
  }
}

Key Patterns

1. Constructor calls super() with panel config, then triggers initial data fetch.
2. fetchData() includes retry logic with exponential backoff and max retry limit.
3. render() builds HTML strings and calls this.setContent(html).
4. destroy() cleans up timers and event listeners.
5. Use showLoading() during initial load (auto-called in constructor).
6. Use showError(msg, retryFn) on failure with retry button.
7. Track error state with _errorState to prevent redundant fetches.
8. Sparkline SVGs use inline <svg> with <polyline> — see sparkline utility.

Sparkline Utility

export function miniSparkline(data: number[] | undefined, change: number | null, w = 50, h = 16): string {
  if (!data || data.length < 2) return '';
  const min = Math.min(...data);
  const max = Math.max(...data);
  const range = max - min || 1;
  const color = change != null && change >= 0 ? 'var(--green)' : 'var(--red)';
  const points = data.map((v, i) => {
    const x = (i / (data.length - 1)) * w;
    const y = h - ((v - min) / range) * (h - 2) - 1;
    return `${x.toFixed(1)},${y.toFixed(1)}`;
  }).join(' ');
  return `<svg width="${w}" height="${h}" viewBox="0 0 ${w} ${h}"><polyline points="${points}" fill="none" stroke="${color}" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round"/></svg>`;
}

Additional Examples

Dynamic Content Update Example

export class NewsPanel extends Panel {
  private newsItems: NewsItem[] = [];

  constructor() {
    super({ id: 'news', title: 'Latest News' });
    this.fetchNews();
    setInterval(() => this.fetchNews(), 300_000); // Update every 5 minutes
  }

  private async fetchNews(): Promise<void> {
    if (this.isFetching) return;
    this.setFetching(true);
    try {
      const news = await fetchNewsItems();
      this.newsItems = news;
      this.renderNews();
    } catch (err) {
      this.showError('Failed to load news', () => this.fetchNews());
    } finally {
      this.setFetching(false);
    }
  }

  private renderNews(): void {
    const items = this.newsItems.map(item => `
      <div class="news-item">
        <h3 class="news-title">${item.title}</h3>
        <p class="news-summary">${item.summary}</p>
        <span class="news-time">${formatTime(item.time)}</span>
      </div>
    `).join('');
    this.setContent(`<div class="news-list">${items}</div>`);
  }
}

Error Handling with State Management

export class WeatherPanel extends Panel {
  private lastSuccessfulFetch: Date | null = null;

  constructor() {
    super({ id: 'weather', title: 'Weather Forecast' });
    this.fetchWeather();
  }

  private async fetchWeather(): Promise<void> {
    if (this.isFetching) return;
    this.setFetching(true);
    try {
      const weather = await fetchWeatherData();
      this.lastSuccessfulFetch = new Date();
      this.renderWeather(weather);
    } catch (err) {
      const message = this.lastSuccessfulFetch
        ? `Failed to update (last success: ${this.lastSuccessfulFetch.toLocaleTimeString()})`
        : 'Failed to load initial data';
      this.showError(message, () => this.fetchWeather());
    } finally {
      this.setFetching(false);
    }
  }

  private renderWeather(weather: WeatherData): void {
    const html = `
      <div class="weather-current">
        <span class="weather-temp">${weather.temp}°</span>
        <span class="weather-desc">${weather.description}</span>
      </div>
      <div class="weather-forecast">
        ${weather.forecast.map(day => `
          <div class="weather-day">
            <span class="weather-day-name">${day.day}</span>
            <span class="weather-day-temp">${day.high}°/${day.low}°</span>
          </div>
        `).join('')}
      </div>
    `;
    this.setContent(html);
  }
}