blECSd Core Library Skill

blECSd is a modern, high-performance terminal UI library built on TypeScript and ECS (Entity Component System) architecture using bitecs. It is a ground-up rewrite of the original blessed node library, NOT backwards-compatible. Version: 0.7.0. Node.js >= 22.0.0.

Hard Rules (Non-Negotiable)

1. Purely Functional, No OOP

BANNED: class, this, new (except Map/Set/Error), prototype manipulation, inheritance.

// WRONG
class MyWidget { private x: number; constructor(x: number) { this.x = x; } }

// CORRECT
interface MyWidget { readonly x: number; }
function createMyWidget(x: number): MyWidget { return { x }; }

2. No Direct bitecs Imports

Only three files may import from 'bitecs': src/core/ecs.ts, src/core/world.ts, src/core/types.ts. Everything else imports from 'blecsd' (external) or '../core/ecs' (internal).

3. Library-First Design

Users control their own world and update loop. All functions take world as a parameter. Never own a global world.

4. Input Priority

INPUT phase is always first in the update loop. Cannot be reordered. All pending input is processed every frame.

5. Early Returns and Guard Clauses

Handle errors first, happy path last. Max nesting 2-3 levels.

6. File Size Limits

• Component files: max 200 lines
• Widget files: max 300 lines per sub-file
• All other source files: max 500 lines

7. Strict TypeScript

• All functions have explicit return types
• No any (use unknown + type guards)
• Prefer readonly arrays and objects
• Branded types for IDs

Architecture

Update Loop Phases (in order)

1. INPUT (always first, immutable position) — keyboard/mouse
2. EARLY_UPDATE — pre-processing, state transitions
3. UPDATE — main game/app logic
4. LATE_UPDATE — post-processing, cleanup
5. ANIMATION — physics, springs, tweens, momentum scrolling
6. LAYOUT — positions, sizes, constraints
7. RENDER — write to screen buffer
8. POST_RENDER — cleanup, telemetry

Where Does Logic Go?

Rule: Components = data only. Systems = logic. Never put business logic in component files.

API Surface (Three Tiers)

Tier 2: Subpath Imports (Recommended)

Full module access — the default for all applications:

import { position, content, scroll } from 'blecsd/components';
import { animationSystem, collisionSystem } from 'blecsd/systems';
import { box, tabs, modal, flexbox } from 'blecsd/widgets';
import { createDoubleBuffer, createProgram } from 'blecsd/terminal';
import { BoxConfigSchema } from 'blecsd/schemas';
import { renderText, wrapText } from 'blecsd/utils';
import { enableDebugOverlay } from 'blecsd/debug';
import { queueKeyEvent } from 'blecsd/input';
import { createViState, processViKey } from 'blecsd/input';

Tier 1: Curated Top-Level ('blecsd')

~80 exports for small scripts and quick prototypes. See [API Reference](./ref/api-surface.md).

Tier 3: Namespace Objects (Preferred for Complex Apps)

Frozen plain objects grouping related functions:

import { position, content, dimensions, border } from 'blecsd/components';

position.set(world, eid, 10, 5);
content.set(world, eid, 'Hello');
dimensions.set(world, eid, 40, 10);
border.set(world, eid, { type: 'line' });

Quick Start with createApp()

The recommended way to bootstrap a blECSd application (added in v0.7.0):

import { createApp } from 'blecsd';
import { createBoxEntity, createTextEntity } from 'blecsd/core';

const app = await createApp({ fullscreen: true, fps: 30 });

// app.world — ECS world
// app.program — terminal input handling
// app.cols, app.rows — terminal dimensions
// app.render() — run one frame
// app.shutdown() — clean exit
// app.start() — start render loop (returns stop fn)

const panel = createBoxEntity(app.world, {
  x: 2, y: 1, width: 40, height: 12,
  border: { type: 1, top: true, bottom: true, left: true, right: true },
});

createTextEntity(app.world, {
  x: 4, y: 2, text: 'My Dashboard', parent: panel,
});

app.program.on('key', (e) => { if (e.name === 'q') app.shutdown(); });
app.start();

createApp Options

Other DX Helpers

Reference Documents

Detailed API surfaces are split into reference files to keep this skill focused:

• [API Surface Reference](./ref/api-surface.md) — Full Tier 1 exports, component namespaces, widget namespaces, systems list
• [Terminal & Server Reference](./ref/terminal-server.md) — Terminal control, server-side (SSH/Telnet/WebSocket), process utils, custom streams, vi mode
• [Graphics & Media Reference](./ref/graphics-media.md) — Graphics manager, braille canvas, vector-to-pixel bridge, image widgets

Common Patterns

Using the Scheduler

import { createWorld, createScheduler, PhaseType } from 'blecsd';

const world = createWorld();
const scheduler = createScheduler();

scheduler.register(inputSystem, PhaseType.INPUT);
scheduler.register(layoutSystem, PhaseType.LAYOUT);
scheduler.register(renderSystem, PhaseType.RENDER);
scheduler.register(outputSystem, PhaseType.POST_RENDER);

function tick() {
  scheduler.run(world);
  requestAnimationFrame(tick);
}
tick();

Custom System

import { defineQuery, defineSystem, hasComponent } from 'blecsd';
import { Position, Velocity } from 'blecsd/components';

const movingQuery = defineQuery([Position, Velocity]);

function createMovementSystem() {
  return defineSystem((world) => {
    for (const eid of movingQuery(world)) {
      Position.x[eid] += Velocity.x[eid];
      Position.y[eid] += Velocity.y[eid];
    }
    return world;
  });
}

Widget API Pattern

import { createBox, setBoxContent, isBox } from 'blecsd/widgets';

const box = createBox(world, {
  position: { x: 0, y: 0 },
  dimensions: { width: '100%', height: '100%' },
  border: { type: 'line', fg: 0x00ff00 },
  padding: { top: 1, left: 2 },
  content: 'Initial content',
});

setBoxContent(world, box, 'Updated content');
if (isBox(world, box)) { /* ... */ }

Keyboard Shortcuts

Global: Tab (focus next), Shift+Tab (focus prev), Escape (blur). Lists: Up/k, Down/j, Home/g, End/G, PageUp/Down, Enter (select), / (search). Text input: Ctrl+A (start), Ctrl+E (end), Ctrl+U (delete to start), Ctrl+K (delete to end), Ctrl+W (delete word).

Error Handling

import { ok, err, isOk, isErr, map } from 'blecsd/errors';

function parseConfig(raw: unknown): Result<Config, ValidationError> {
  const result = ConfigSchema.safeParse(raw);
  if (!result.success) return err(createValidationError(result.error));
  return ok(result.data);
}

Error categories: validation, terminal, system, entity, component, input, render, config, internal.

Testing

import { describe, it, expect } from 'vitest';
import { createWorld, addEntity, addComponent, hasComponent } from 'blecsd';

describe('movement system', () => {
  it('updates position from velocity', () => {
    const world = createWorld();
    const eid = addEntity(world);
    addComponent(world, eid, Position);
    addComponent(world, eid, Velocity);
    Position.x[eid] = 0;
    Velocity.x[eid] = 5;

    movementSystem(world);

    expect(Position.x[eid]).toBe(5);
  });
});

⚠️ Critical: renderSystem Does NOT Render Text Content

The base renderSystem only renders borders and backgrounds. The renderContent() function is a no-op placeholder (see src/systems/renderSystem.ts:341). This means:

• setContent(world, eid, "text") stores data but nothing appears on screen
• createTextEntity(world, { text: "Hello" }) creates an invisible text entity

All official examples use raw ANSI rendering via writeRaw() from blecsd/systems. For TUI apps that need visible text, use:

import { writeRaw, cursorHome, enterAlternateScreen, hideCursor, setOutputStream } from "blecsd/systems";
import { clearScreen } from "blecsd";

setOutputStream(process.stdout);
enterAlternateScreen();
hideCursor();

// Render with ANSI escape codes
writeRaw(`\x1b[${row};${col}H\x1b[38;2;${r};${g};${b}mHello blECSd!`);

⚠️ Critical: Dev Server Setup for TUI Apps

NEVER use tsx watch or nodemon for interactive TUI apps. They steal or pipe stdin, breaking process.stdin.setRawMode(true). Symptoms: keypress restarts app, ANSI escape codes echo on screen, arrow keys don't work.

Use this dev.mjs pattern instead:

import { spawn } from "node:child_process";
import { watch } from "node:fs";
let child = null, restarting = false;
function start() {
  child = spawn("npx", ["tsx", "src/index.ts"], {
    stdio: "inherit",  // Child gets actual TTY
    env: { ...process.env },
  });
  child.on("exit", (code) => {
    child = null;
    if (restarting) { restarting = false; start(); }
    else process.exit(code || 0);
  });
}
let debounce = null;
watch("src", { recursive: true }, (_, f) => {
  if (!f?.endsWith(".ts") || debounce) return;
  debounce = setTimeout(() => { debounce = null; }, 500);
  if (child) { restarting = true; child.kill("SIGTERM"); } else start();
});
start();

{ "scripts": { "dev": "node dev.mjs" } }

⚠️ Critical: Use Entity Factories, Not addEntity()

addEntity(world) creates a bare entity with zero components — no Renderable, no Position, no Dimensions. It will be completely invisible with no error. Always use createBoxEntity(), createTextEntity(), or other factories from blecsd/core.

Common Anti-Patterns

1. Using classes — All code must be functional.
2. Importing from bitecs directly — Always import from blecsd or ../core/ecs.
3. Putting logic in component files — Components are data only.
4. Deep nesting — Use guard clauses and early returns.
5. Using any — Use unknown with type guards.
6. Missing Zod validation at boundaries — All config objects need Zod schemas.
7. Forgetting barrel exports — Update the module's index.ts when adding exports.
8. Processing input outside INPUT phase — All input goes through inputSystem.
9. Using addEntity() for UI elements — Use createBoxEntity() etc. Bare entities lack Renderable and are invisible.
10. Using tsx watch for TUI apps — Use dev.mjs with stdio: "inherit" spawn. See warning above.
11. Expecting renderSystem to show text — It only renders borders/backgrounds. Use writeRaw() for text.

Module Ownership (Ambiguous Names)

Development Commands

pnpm install          # Install dependencies
pnpm dev              # Development mode
pnpm build            # Build (catches issues tests miss)
pnpm test             # Run tests
pnpm test:watch       # Watch mode
pnpm lint             # Biome linter
pnpm lint:fix         # Auto-fix lint
pnpm typecheck        # TypeScript type check

Performance Tips

• Cache queries: const myQuery = defineQuery([Position, Velocity]) once, reuse everywhere.
• Batch component reads in one pass per entity.
• Use dirty tracking: only re-render changed entities.
• Virtualize large lists with createVirtualizedList.
• Use double buffering (createDoubleBuffer) for flicker-free rendering.
• Avoid allocations in hot loops.
• Use frameBudgetSystem to monitor frame times.

Add-on Packages