# Typescript Style

> Guides ccusage TypeScript and JavaScript reading, writing, and review. Use before opening or editing .ts, .tsx, .js, or .jsx files, including mocks, fixtures, satisfies, or suppressions.

- Skill: `tuyv/typescript-style` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add tuyv/typescript-style`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tuyv/typescript-style/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tuyv (https://skillmd.com/u/tuyv)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tuyv/typescript-style

---


# TypeScript Style

## Prefer `satisfies` Over `as`

Avoid `as` type assertions and especially `as any`. Use `satisfies` when checking object literals, mocks, config objects, fixture data, and expected rows.

Bad:

```ts
const config = { port: 3000, host: 'localhost' } as ServerConfig;
const ctx = { fetchDirent: vi.fn(), addReadHistory: vi.fn() } as any;
```

Good:

```ts
const config = { port: 3000, host: 'localhost' } satisfies ServerConfig;
const ctx = { fetchDirent: vi.fn(), addReadHistory: vi.fn() } satisfies MockContext;
```

## Use `as const satisfies`

Use `as const satisfies` for static literal data when preserving exact literal values or readonly tuples helps TypeScript catch mistakes.

Good:

```ts
const ROUTES = {
	home: '/',
	about: '/about',
} as const satisfies Record<string, string>;
```

Good for table-driven cases:

```ts
const reportCases = [
	{ type: 'daily', period: '2026-05-16' },
	{ type: 'monthly', period: '2026-05' },
] as const satisfies readonly ReportCase[];
```

## Acceptable `as`

Use `as` only when `satisfies` cannot express the operation, such as narrowing data returned from an external untyped boundary or adapting to an API that requires a nominal type. Keep it local and avoid `as any`.

## Prefer `@ts-expect-error`

Use `@ts-expect-error` instead of `@ts-ignore`, and include a short explanation. `@ts-expect-error` fails when the underlying type error disappears, so stale suppressions are caught.

Bad:

<!-- eslint-skip -->

```ts
// @ts-ignore
import privateApi from 'some-package/private-api';
```

## Exhaustive Switches

Use `satisfies never` after switches over discriminated unions or literal unions so new variants fail typecheck until the branch is handled.

Good:

```ts
switch (agent) {
	case 'claude':
		return loadClaude();
	case 'codex':
		return loadCodex();
	default:
		// exhaustiveness check for new agents added to the union
		agent satisfies never;
		throw new Error(`Unsupported agent: ${agent}`);
}
```

Good:

```ts
// @ts-expect-error Private runtime API has no public type declaration.
import privateApi from 'some-package/private-api';
```