Scaffold ESM TypeScript rewrite as @larvit/smpp

test/result.test.ts

@@ -0,0 +1,29 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import type { Result } from '../src/result.ts';
+
+function parse(input: string): Result<{ value: number }> {
+	const value = Number(input);
+
+	if (Number.isNaN(value)) return { err: new Error('not a number') };
+
+	return { value };
+}
+
+test('destructuring a result narrows on err', () => {
+	const { err, value } = parse('42');
+
+	if (err) {
+		assert.fail('should have parsed');
+	}
+
+	// Fails to compile if narrowing does not remove undefined from value.
+	assert.equal(value.toFixed(0), '42');
+});
+
+test('a failed result carries err and nothing else', () => {
+	const { err, value } = parse('nope');
+
+	assert.ok(err instanceof Error);
+	assert.equal(value, undefined);
+});