Resilient Panel — Unified Pattern

Create dashboard panel components using vanilla TypeScript (no framework, no JSX). Each panel is a class extending a Panel base class and optionally wraps its data source with a CircuitBreaker.

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Architecture Overview

Panel (base class)
├── element: HTMLElement (.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)

Resilience layers (choose what you need)

Layer	File	When to use
isFetching guard	Panel base class	Always — prevents concurrent duplicate requests
Exponential-backoff retry	Panel.fetchWithRetry()	When you own the fetch call and want per-request retry before surfacing an error
CircuitBreaker	src/utils/circuit-breaker.ts	When a service may fail repeatedly; stops hammering the API and serves stale cache instead
localStorage persistence	Panel.saveState() / loadState()	When you want collapsed/size state to survive page reloads

Wrapping an existing service call with CircuitBreaker is a one-liner:

// Before
const data = await fetchStockQuotes();
// After — adds caching + cooldown with zero refactor
const data = await breaker.execute(() => fetchStockQuotes(), []);

---

File 1 — 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;

  // Retry state (used by fetchWithRetry)
  protected retryAttempts = 0;
  protected retryDelay = 1000; // ms; doubles each attempt
  private maxRetries = 3;

  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 API ───────────────────────────────────────────────────────

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

  public showLoading(message = 'Loading...'): void {
    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.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.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'); }

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

  // ── Protected helpers ────────────────────────────────────────────────

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

  /**
   * Fetch `url` with exponential-backoff retry (up to maxRetries attempts).
   * Reset `this.retryAttempts` and `this.retryDelay` before calling if you
   * want a fresh retry sequence (e.g. from a user-initiated Retry button).
   */
  protected async fetchWithRetry(url: string): Promise<unknown> {
    while (this.retryAttempts < this.maxRetries) {
      try {
        const response = await fetch(url);
        if (!response.ok) throw new Error(`HTTP ${response.status}`);
        return await response.json();
      } catch (error) {
        this.retryAttempts++;
        if (this.retryAttempts >= this.maxRetries) {
          throw new Error(
            `Failed after ${this.maxRetries} attempts: ${(error as Error).message}`
          );
        }
        await new Promise(resolve => setTimeout(resolve, this.retryDelay));
        this.retryDelay *= 2;
      }
    }
  }

  /** Persist expanded/collapsed state and dimensions to localStorage. */
  public saveState(): void {
    localStorage.setItem(`panelState_${this.panelId}`, JSON.stringify({
      isExpanded: !this.element.classList.contains('collapsed'),
      width: this.element.style.width,
      height: this.element.style.height,
    }));
  }

  /** Restore state saved by saveState(). */
  public loadState(): void {
    const raw = localStorage.getItem(`panelState_${this.panelId}`);
    if (!raw) return;
    const { isExpanded, width, height } = JSON.parse(raw) as {
      isExpanded: boolean; width: string; height: string;
    };
    if (!isExpanded) this.element.classList.add('collapsed');
    if (width)  this.element.style.width  = width;
    if (height) this.element.style.height = height;
  }
}

---

File 2 — src/utils/circuit-breaker.ts

interface CircuitState {
  failures: number;
  cooldownUntil: number;
}

interface CacheEntry<T> {
  data: T;
  timestamp: number;
}

export interface CircuitBreakerOptions {
  name: string;
  maxFailures?: number;   // default 2
  cooldownMs?: number;    // default 5 min
  cacheTtlMs?: number;    // default 10 min
}

export class CircuitBreaker<T> {
  private state: CircuitState = { failures: 0, cooldownUntil: 0 };
  private cache: CacheEntry<T> | null = null;
  private readonly name: string;
  private readonly maxFailures: number;
  private readonly cooldownMs: number;
  private readonly cacheTtlMs: number;

  constructor(options: CircuitBreakerOptions) {
    this.name        = options.name;
    this.maxFailures = options.maxFailures ?? 2;
    this.cooldownMs  = options.cooldownMs  ?? 5 * 60 * 1000;
    this.cacheTtlMs  = options.cacheTtlMs  ?? 10 * 60 * 1000;
  }

  isOnCooldown(): boolean {
    if (Date.now() < this.state.cooldownUntil) return true;
    if (this.state.cooldownUntil > 0) {
      this.state = { failures: 0, cooldownUntil: 0 };
    }
    return false;
  }

  getCached(): T | null {
    if (this.cache && Date.now() - this.cache.timestamp < this.cacheTtlMs) {
      return this.cache.data;
    }
    return null;
  }

  recordSuccess(data: T): void {
    this.state = { failures: 0, cooldownUntil: 0 };
    this.cache = { data, timestamp: Date.now() };
  }

  recordFailure(): void {
    this.state.failures++;
    if (this.state.failures >= this.maxFailures) {
      this.state.cooldownUntil = Date.now() + this.cooldownMs;
      console.warn(`[${this.name}] Circuit open — cooldown ${this.cooldownMs / 1000}s`);
    }
  }

  /**
   * Execute fn. On success, caches result. On failure, records failure and
   * returns defaultValue. While on cooldown or cache is fresh, skips fn
   * entirely and returns cached/default value.
   */
  async execute<R extends T>(fn: () => Promise<R>, defaultValue: R): Promise<R> {
    if (this.isOnCooldown()) {
      const cached = this.getCached();
      return (cached as R) ?? defaultValue;
    }
    const cached = this.getCached();
    if (cached !== null) return cached as R;
    try {
      const result = await fn();
      this.recordSuccess(result);
      return result;
    } catch (e) {
      console.error(`[${this.name}] Request failed:`, e);
      this.recordFailure();
      return defaultValue;
    }
  }
}

export function createCircuitBreaker<T>(
  options: CircuitBreakerOptions
): CircuitBreaker<T> {
  return new CircuitBreaker<T>(options);
}

---

File 3 — src/utils/sparkline.ts

/**
 * Returns an inline SVG sparkline string, or '' if data is too short.
 * Embed the return value directly in HTML template strings.
 */
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>`;
}

---

Creating a Concrete Panel

Option A — Wrapping an existing project service with CircuitBreaker (preferred)

Use this when the project already has a fetchStockQuotes() (or similar) function. The circuit breaker is a one-liner wrapper — no refactoring of the service needed.

import { Panel } from './Panel';
import { createCircuitBreaker } from '../utils/circuit-breaker';
import { fetchStockQuotes } from '../services/stocks'; // existing project service

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

// Module-level breaker — shared across re-renders, persists cache between fetches
const breaker = createCircuitBreaker<StockQuote[]>({
  name: 'StockMarket',
  cacheTtlMs: 60_000,
  cooldownMs: 5 * 60_000,
});

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

  constructor() {
    super({ id: 'stocks', title: 'Stock Market', showCount: true });
    this.loadState();       // restore collapsed/size from localStorage
    this.fetchData();
    this.refreshTimer = setInterval(() => this.fetchData(), 60_000);

    // Optional: manual refresh button in header
    const btn = document.createElement('button');
    btn.className = 'panel-refresh-btn';
    btn.textContent = 'Refresh';
    btn.addEventListener('click', () => this.fetchData());
    this.header.appendChild(btn);
  }

  private async fetchData(): Promise<void> {
    if (this.isFetching) return;
    this.setFetching(true);
    try {
      // One-liner: wrap existing service call with circuit breaker
      const quotes = await breaker.execute(() => fetchStockQuotes(), []);
      this.render(quotes);
      this.setCount(quotes.length);
      this.saveState();
    } catch (err) {
      this.showError(
        `Failed to load stock data: ${(err as Error).message}`,
        () => 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'}">
          ${q.change != null ? (q.change >= 0 ? '+' : '') + q.change.toFixed(2) + '%' : '--'}
        </span>
      </div>`).join('');
    this.setContent(`<div class="stock-list">${rows}</div>`);
  }

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

Option B — Direct fetch with fetchWithRetry (no existing service layer)

Use this when there is no project service and you want per-request exponential backoff:

import { Panel } from './Panel';

interface NewsArticle {
  id: string;
  title: string;
  url: string;
}

export class NewsPanel extends Panel {
  private refreshTimer: ReturnType<typeof setInterval> | null = null;

  constructor() {
    super({ id: 'news', title: 'Live News', showCount: true });
    this.fetchData();
    this.refreshTimer = setInterval(() => this.fetchData(), 120_000);
  }

  private async fetchData(): Promise<void> {
    if (this.isFetching) return;
    this.setFetching(true);
    try {
      // fetchWithRetry handles up to 3 attempts with exponential backoff
      const articles = await this.fetchWithRetry('/api/news') as NewsArticle[];
      this.render(articles);
      this.setCount(articles.length);
    } catch (err) {
      this.showError(
        `News unavailable: ${(err as Error).message}`,
        () => {
          // Reset retry counters before user-initiated retry
          this.retryAttempts = 0;
          this.retryDelay = 1000;
          this.fetchData();
        }
      );
    } finally {
      this.setFetching(false);
    }
  }

  private render(articles: NewsArticle[]): void {
    const items = articles.map(a =>
      `<div class="news-item"><a href="${a.url}">${a.title}</a></div>`
    ).join('');
    this.setContent(`<div class="news-list">${items}</div>`);
  }

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

Option C — Plain fetch, no resilience layer

For non-critical panels or mock/dev data where retries add no value:

private async fetchData(): Promise<void> {
  if (this.isFetching) return;
  this.setFetching(true);
  try {
    const resp = await fetch('/api/config');
    if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
    const data = await resp.json();
    this.render(data);
  } catch (err) {
    this.showError('Could not load config', () => this.fetchData());
  } finally {
    this.setFetching(false);
  }
}

---

Decision Guide — Which resilience layer?

Does the project have an existing fetchX() service function?
  YES -> wrap it: breaker.execute(() => fetchX(), defaultValue)     <- Option A
  NO  -> does the endpoint fail intermittently under normal load?
           YES -> use fetchWithRetry() for per-request retry         <- Option B
           NO  -> plain fetch() with showError/retry is enough       <- Option C

Is the endpoint unreliable or rate-limited for extended periods?
  YES -> add CircuitBreaker (prevents hammering during outages)
  NO  -> fetchWithRetry or plain fetch is sufficient

Do you need panel state (size, collapsed) to survive page reloads?
  YES -> call this.loadState() in constructor (after super())
         call this.saveState() after a successful render

---

Protected API Reference

All members below are accessible from subclasses without importing or inspecting Panel.ts directly:

Member / Method	Type	Description
element	HTMLElement	Outer container div (.panel)
header	HTMLElement	Header bar div — append extra controls here
content	HTMLElement	Content area div (.panel-content)
countEl	HTMLElement | null	Count badge, or null if showCount not set
panelId	string	The id from PanelOptions
retryAttempts	number	Current retry count for fetchWithRetry
retryDelay	number	Current delay (ms) for fetchWithRetry; reset to 1000 before retry
isFetching	boolean (getter)	true while an async fetch is in progress
setFetching(v)	void	Set the fetching guard flag
showLoading(msg?)	void	Replace content with a loading spinner
showError(msg?, onRetry?)	void	Replace content with an error state and optional retry button
setContent(html)	void	Set raw HTML into the content area
setCount(n)	void	Update the count badge (no-op if countEl is null)
fetchWithRetry(url)	Promise<unknown>	Fetch with exponential-backoff retry (3 attempts)
saveState()	void	Persist expanded/size state to localStorage
loadState()

Example — appending a button to the header in a subclass:

constructor() {
  super({ id: 'insights', title: 'Insights', className: 'panel-wide' });
  const btn = document.createElement('button');
  btn.className = 'panel-refresh-btn';
  btn.textContent = 'Refresh';
  btn.addEventListener('click', () => this.generate());
  this.header.appendChild(btn); // 'header' is protected — safe to use here
}

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Key Patterns (checklist)

1. Constructor: call super() with PanelOptions, optionally call loadState(), then trigger initial data fetch.
2. fetchData(): async, use isFetching guard, wrap service calls with breaker.execute() when available.
3. render(): build HTML strings, call this.setContent(html), then this.saveState().
4. Error recovery: showError(message, () => this.fetchData()) — retry button is wired automatically.
5. destroy(): clear all setInterval / setTimeout handles, then call super.destroy().
6. Sparklines: import miniSparkline from src/utils/sparkline.ts, embed return value directly in HTML template strings.
7. Header controls: this.header.appendChild(el) — header is protected, safe to use in subclasses.

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localStorage Utilities (optional helpers)

See examples/localStorageUtils.ts for typed wrappers when you need to persist additional per-panel data beyond the built-in size/collapsed state (e.g. user filter selections, last-viewed item).