typescript-style

name	typescript-style
description	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.
paths	["**/*.ts","**/*.tsx","**/*.js","**/*.jsx"]
globs	*.ts,*.tsx,*.js,*.jsx

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:

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

Good:

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:

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

Good for table-driven cases:

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:

// @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:

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