xtermjs

xterm.js Development Skill

xterm.js (@xterm/xterm) is a full-featured terminal emulator that runs in the browser or Electron. It is NOT a shell — it must be connected to a backend process (e.g. via node-pty + WebSocket) to execute commands.

---

1. Installation

npm install @xterm/xterm
# Required addons (install only what you need):
npm install @xterm/addon-fit @xterm/addon-attach @xterm/addon-web-links \
            @xterm/addon-search @xterm/addon-webgl @xterm/addon-clipboard \
            @xterm/addon-web-fonts @xterm/addon-unicode11

Always import the CSS:

<link rel="stylesheet" href="node_modules/@xterm/xterm/css/xterm.css" />

Or in JS/TS:

import '@xterm/xterm/css/xterm.css';

---

2. Basic Setup

import { Terminal } from '@xterm/xterm';

const term = new Terminal({
  cols: 80,
  rows: 24,
  cursorBlink: true,
  scrollback: 5000,
  fontFamily: '"Cascadia Code", Menlo, monospace',
  fontSize: 14,
  theme: {
    background: '#1e1e1e',
    foreground: '#d4d4d4',
    cursor: '#d4d4d4',
  },
});

term.open(document.getElementById('terminal')!);
term.write('Hello from \x1B[1;32mxterm.js\x1B[0m\r\n$ ');

Critical: Always call term.open(element) AFTER the element is in the DOM. Use \r\n (not just \n) for newlines when writing directly.

---

3. Official Addons

Addon	Package	Purpose
FitAddon	@xterm/addon-fit	Resize terminal to fill its container
AttachAddon	@xterm/addon-attach	Connect to a WebSocket backend
SearchAddon	@xterm/addon-search	In-terminal text search
WebglAddon	@xterm/addon-webgl	GPU-accelerated WebGL2 renderer
WebLinksAddon	@xterm/addon-web-links	Clickable URLs
ClipboardAddon	@xterm/addon-clipboard	Browser clipboard integration
WebFontsAddon	@xterm/addon-web-fonts	Wait for web fonts before rendering
Unicode11Addon	@xterm/addon-unicode11	Unicode 11 character width support
LigaturesAddon	@xterm/addon-ligatures	Font ligature support (canvas renderer only)

Loading addons

import { FitAddon } from '@xterm/addon-fit';
import { WebglAddon } from '@xterm/addon-webgl';
import { WebLinksAddon } from '@xterm/addon-web-links';
import { SearchAddon } from '@xterm/addon-search';

const fitAddon = new FitAddon();
const searchAddon = new SearchAddon();

term.loadAddon(fitAddon);
term.loadAddon(searchAddon);
term.loadAddon(new WebLinksAddon());

term.open(document.getElementById('terminal')!);

// WebGL: load AFTER open(), handle fallback
try {
  const webgl = new WebglAddon();
  webgl.onContextLoss(() => webgl.dispose()); // fallback on context loss
  term.loadAddon(webgl);
} catch {
  console.warn('WebGL2 not available, falling back to canvas renderer');
}

fitAddon.fit();

---

4. FitAddon: Responsive Resizing

FitAddon resizes cols/rows to fit the container's pixel dimensions.

// Resize on window resize
const ro = new ResizeObserver(() => fitAddon.fit());
ro.observe(document.getElementById('terminal')!);

// Or with window resize event (less precise):
window.addEventListener('resize', () => fitAddon.fit());

Gotcha: Container must have explicit dimensions (CSS width/height). FitAddon returns undefined if the container has zero size.

---

5. Backend Integration: node-pty + WebSocket

Frontend (AttachAddon)

import { AttachAddon } from '@xterm/addon-attach';

const socket = new WebSocket('ws://localhost:3000/ws');
const attachAddon = new AttachAddon(socket);
term.loadAddon(attachAddon);

// Sync terminal resize with PTY on the server
term.onResize(({ cols, rows }) => {
  socket.send(JSON.stringify({ type: 'resize', cols, rows }));
});

Backend (Node.js + node-pty)

import * as pty from 'node-pty';
import { WebSocketServer } from 'ws';

const wss = new WebSocketServer({ port: 3000 });

wss.on('connection', (ws) => {
  const shell = process.platform === 'win32' ? 'powershell.exe' : 'bash';
  const ptyProcess = pty.spawn(shell, [], {
    name: 'xterm-color',
    cols: 80,
    rows: 24,
    env: process.env,
  });

  ptyProcess.onData((data) => ws.send(data));
  ws.on('message', (msg) => {
    const parsed = JSON.parse(msg.toString());
    if (parsed.type === 'resize') {
      ptyProcess.resize(parsed.cols, parsed.rows);
    } else {
      ptyProcess.write(msg.toString());
    }
  });
  ws.on('close', () => ptyProcess.kill());
});

For simple bidirectional use (no resize), AttachAddon handles piping automatically; manual term.onData / pty.onData wiring is only needed for custom protocols.

---

6. Manual Data Wiring (without AttachAddon)

// PTY → terminal
ptyProcess.onData((data) => term.write(data));

// Terminal → PTY (user keystrokes)
term.onData((data) => ptyProcess.write(data));

// Binary events (e.g., certain mouse reports)
term.onBinary((data) => ptyProcess.write(data));

---

7. Theming

Pass an ITheme object in options or use term.options.theme = {...} at runtime:

const darkTheme = {
  background: '#0d1117',
  foreground: '#c9d1d9',
  cursor: '#58a6ff',
  cursorAccent: '#0d1117',
  selectionBackground: '#264f78',
  black:   '#484f58', red:     '#ff7b72', green:   '#3fb950', yellow:  '#d29922',
  blue:    '#58a6ff', magenta: '#bc8cff', cyan:    '#39c5cf', white:   '#b1bac4',
  brightBlack:   '#6e7681', brightRed:   '#ffa198', brightGreen:  '#56d364',
  brightYellow:  '#e3b341', brightBlue:  '#79c0ff', brightMagenta:'#d2a8ff',
  brightCyan:    '#56d4dd', brightWhite: '#f0f6fc',
};

// Apply at construction
const term = new Terminal({ theme: darkTheme });

// Or update at runtime
term.options.theme = darkTheme;

---

8. Key Event Handling

// Intercept keys before terminal processes them
// Return false to suppress, true to allow
term.attachCustomKeyEventHandler((ev: KeyboardEvent) => {
  // Example: Ctrl+Shift+C → copy
  if (ev.ctrlKey && ev.shiftKey && ev.key === 'C') {
    document.execCommand('copy');
    return false; // don't pass to terminal
  }
  return true;
});

---

9. Search (SearchAddon)

// Find next/previous
searchAddon.findNext('search term', {
  regex: false,
  wholeWord: false,
  caseSensitive: false,
  incremental: false,       // true = highlight while typing
  decorations: {
    matchBackground: '#ffff0040',
    matchBorder: '#ffff00',
    matchOverviewRuler: '#ffff00',
    activeMatchBackground: '#ff000080',
    activeMatchBorder: '#ff0000',
    activeMatchColorOverviewRuler: '#ff0000',
  },
});
searchAddon.findPrevious('search term');

---

10. Decorations and Markers

// Add a marker on the current row
const marker = term.registerMarker(0); // 0 = current row offset

// Add a decoration (e.g. highlight a line)
const decoration = term.registerDecoration({
  marker,
  overviewRulerOptions: { color: '#ff0000' },
});

decoration?.onRender((element) => {
  element.style.backgroundColor = 'rgba(255,0,0,0.2)';
});

---

11. Parser Hooks (Custom Sequences)

// Register a custom OSC sequence handler (e.g. OSC 1337)
term.parser.registerOscHandler(1337, (data: string) => {
  console.log('Custom OSC 1337 payload:', data);
  return true; // handled
});

// Register a custom CSI sequence handler
term.parser.registerCsiHandler({ final: 'z' }, (params) => {
  console.log('Custom CSI z params:', params);
  return true;
});

---

12. React Integration Pattern

import { useEffect, useRef } from 'react';
import { Terminal } from '@xterm/xterm';
import { FitAddon } from '@xterm/addon-fit';
import '@xterm/xterm/css/xterm.css';

export function XTerminal() {
  const containerRef = useRef<HTMLDivElement>(null);
  const termRef = useRef<Terminal | null>(null);

  useEffect(() => {
    const term = new Terminal({ cursorBlink: true });
    const fitAddon = new FitAddon();
    term.loadAddon(fitAddon);
    term.open(containerRef.current!);
    fitAddon.fit();
    termRef.current = term;

    const ro = new ResizeObserver(() => fitAddon.fit());
    ro.observe(containerRef.current!);

    return () => {
      ro.disconnect();
      term.dispose();
    };
  }, []);

  return <div ref={containerRef} style={{ width: '100%', height: '400px' }} />;
}

---

13. Common Pitfalls

Problem	Cause	Fix
Terminal renders blank / wrong size	FitAddon called before DOM paint	Call fitAddon.fit() after open(), use ResizeObserver
Characters appear wrong width	Unicode handling	Load Unicode11Addon, call term.unicode.activeVersion = '11'
Backspace doesn't work	PTY not connected / wrong escape	Send \x7f or \x08; check PTY termios settings
Copy/paste broken on some browsers	Browser security	Use ClipboardAddon and ensure HTTPS or localhost
WebGL context lost after tab switch	GPU resource reclaim	webgl.onContextLoss(() => webgl.dispose())
Terminal doesn't fill container	Container has no height	Set explicit CSS height on container element
\n without \r causes staircase	Missing carriage return	Use \r\n or enable convertEol: true in options

---

14. Key Terminal Options Reference

const term = new Terminal({
  cols: 80,              // initial columns
  rows: 24,              // initial rows
  cursorBlink: true,     // blinking cursor
  cursorStyle: 'block',  // 'block' | 'underline' | 'bar'
  scrollback: 10000,     // scrollback buffer lines
  tabStopWidth: 4,       // tab stop width
  convertEol: false,     // auto-add \r on \n (avoid for real PTY)
  disableStdin: false,   // read-only mode
  allowProposedApi: false, // enable experimental APIs
  allowTransparency: false, // transparent background support
  windowsMode: false,    // Windows-style line ending behavior
  macOptionIsMeta: false, // treat Option key as Meta (macOS)
  rightClickSelectsWord: false,
  fontSize: 14,
  fontFamily: 'monospace',
  fontWeight: 'normal',
  fontWeightBold: 'bold',
  lineHeight: 1.0,
  letterSpacing: 0,
  logLevel: 'info',      // 'debug' | 'info' | 'warn' | 'error' | 'off'
});

---

15. High-Performance Data Handling

When connected to high-throughput backends (LLMs, build tools, log streams), naive patterns cause UI freezes and data corruption.

UTF-8 Boundary Handling

Multi-byte UTF-8 sequences can be split across read chunks from the PTY. The reader must maintain a remainder buffer to reassemble them before passing to term.write().

Coalescing PTY Output with React State

Do NOT synchronize React state on every PTY onData chunk -- hundreds per second during heavy output will freeze the UI.

• Use intermediate buffers to accumulate data
• Use requestAnimationFrame to extract terminal state (cursor position, buffer length, bookmarks) at most 60 times per second
• Never call term.refresh() during active output -- it resets scroll position. Use a debounced timer for manual refreshes

Cursor Ghosting During Rapid Output

During fast PTY output (LLM streaming, build logs), the cursor appears and disappears at random positions creating a flickering/ghost effect.

Fix: Hide cursor during ALL PTY writes with term.write('\x1b[?25l'), then use a debounced timer (80-100ms) that restores the cursor only when output stabilizes. Consolidate cursor movement detection into a single regex instead of multiple sequential string scans.

Viewport Index Calculation

To read the correct buffer line, always use buffer.baseY + buffer.cursorY for the absolute index. Using cursorY alone reads from the top of the scrollback, not the current viewport row.

Throttling Expensive UI Updates

Components that re-render based on terminal state (minimap, bookmarks, line counters) must throttle updates:

• Throttle minimap/canvas re-renders to 150ms with a trailing render guarantee
• Cap hardware pixel dimensions for canvas elements at high DPR to avoid GPU limits
• Coalesce bookmark sync via requestAnimationFrame instead of per-chunk updates

---

16. Stream Interception & ANSI Manipulation

Before calling term.write(), the data stream may need preprocessing:

ConPTY Windows Quirks

Windows ConPTY corrupts supplementary plane characters (emoji) when passed through CreateProcessW. Strip or replace emoji from text before passing to ConPTY to prevent lone surrogates corrupting the data stream.

When the shell resolves to a .cmd/.bat shim, cmd.exe metacharacters in arguments can cause injection. Use cmd.exe-specific quoting: "" for embedded quotes and %% for percent signs.

Paste Sanitization

Terminal control characters in pasted text can inject commands or corrupt output. Sanitize pasted text by filtering terminal output control characters. Block the native paste event with preventDefault() to prevent double-paste, and add a debounce guard for rapid paste events.

Banner/Output Interception

To replace or filter backend output (e.g. replacing a startup banner with custom content):

1. Buffer initial PTY output until a known delimiter is detected
2. Replace the buffered content with custom output
3. On screen clear events (ESC[2J), re-activate the interception to handle redraws (e.g. on window resize)
4. During post-replacement cooldown, filter positioning sequences (CUP) that would overwrite the replacement, but preserve printable content

Prefer HTML overlay approaches (React component with CSS fade-out) over inline ANSI manipulation when possible -- inline approaches conflict with CUP offset tracking.

---

17. Browser Quirks & WebGL Resiliency

Container Initialization

NEVER use display: none on the xterm container during initialization -- cols/rows calculation will fail. Use visibility: hidden instead, or mount the component only when visible.

WebGL Context Loss After Sleep/Standby

A single onContextLoss callback is insufficient. After system sleep/standby, the WebGL context dies silently and the addon callback often does not fire.

Use a 3-layer detection strategy:

1. Addon callback: webglAddon.onContextLoss(() => fallbackToCanvas())
2. DOM event: listen for webglcontextlost on the canvas element generated by the addon
3. Periodic health check: timer that verifies the WebGL context is still valid

All three layers should trigger automatic fallback to the canvas renderer.

Tab Switching

Defer fitAddon.fit() to the next requestAnimationFrame when switching tabs, to give the DOM time to update container dimensions. Fitting immediately causes narrow columns.

Resize Throttling

Throttle terminal resize events to prevent a burst of resize messages to the backend PTY. Also reset PTY size tracking after spawning a new process to avoid stale column counts from the previous session.

---

18. Viewport Virtualization & Coordinate Translation

When injecting custom content (banners, status bars) that the backend PTY does not know about, absolute cursor positioning (CUP sequences) breaks because the backend's row count does not match the frontend's.

Pattern: Coordinate Lying

1. Lie about dimensions: Subtract the offset of injected rows before sending dimensions to the PTY (both at spawn and on resize)
2. Clamp values: Always enforce a minimum (e.g. Math.max(10, rows - OFFSET)) to prevent backend crashes on tiny terminals
3. Intercept and translate: Use a stream middleware to intercept incoming CUP sequences (\x1b[<row>;<col>H), adding the offset back before passing to term.write()

// Example: frontend has 30 rows, custom banner is 12 rows
const CUP_ROW_OFFSET = 12; // derive from banner content, not magic number

// Lie to PTY on spawn and resize
term.onResize(({ cols, rows }) => {
  const virtualRows = Math.max(10, rows - CUP_ROW_OFFSET);
  socket.send(JSON.stringify({ type: 'resize', cols, rows: virtualRows }));
});

Derive the offset from the actual content size (e.g. bannerLines.length) rather than hardcoding magic numbers. When the injected content can change (resize-triggered redraw), re-derive the offset dynamically.

---

19. Disposal

Always dispose to prevent memory leaks:

term.dispose();         // disposes terminal and all loaded addons
fitAddon.dispose();     // or dispose individual addons
marker.dispose();       // markers and decorations
decoration.dispose();

---

References

• API docs: https://xtermjs.org/docs/api/terminal/classes/terminal/
• Addon docs: https://xtermjs.org/docs/guides/using-addons/
• Parser hooks: https://xtermjs.org/docs/guides/hooks/
• GitHub: https://github.com/xtermjs/xterm.js
• node-pty: https://github.com/microsoft/node-pty