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:

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';