test: a failed build leaves dist/ as it was

scripts/build.mjs and its commit message state that a failed build does not touch dist/. That held when checked by
hand, and no test covered it, so a later change to the swap could break it unnoticed.

test/build.test.js runs a copy of the script in a temporary package with one source file and the dist/ of an
earlier build, and compares every file and directory of that package before and after. Two cases: a type error
(the first compiler run fails, TS2322) and "export =" (CommonJS compiles, the ES module run fails with TS1203, so
the staging directory already holds cjs/ and types/).

Checked in a scratch copy: with the cleanup of the staging directory removed both tests fail; with cjs/ and types/
swapped in before the ES module run the second one fails. npm test: 27 pass in 5.7 s.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/build.test.js

@@ -0,0 +1,68 @@
+"use strict";
+// scripts/build.mjs on a package of its own. The script takes src/, the tsconfig files and typescript from where it
+// is, so a copy of it builds the directory it is put in and the tracked dist/ is never written.
+const { test } = require("node:test");
+const assert = require("node:assert/strict");
+const fs = require("node:fs");
+const os = require("node:os");
+const path = require("node:path");
+const { spawnSync } = require("node:child_process");
+
+const ROOT = path.join(__dirname, "..");
+
+// A package directory with the build script, the three tsconfig files, src/index.ts and the dist/ of an earlier
+// build; removed when the test ends.
+function scratchPackage(t, source) {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "alex2node-build-"));
+  t.after(() => fs.rmSync(dir, { recursive: true, force: true }));
+  for (const file of ["scripts/build.mjs", "tsconfig.json", "tsconfig.cjs.json", "tsconfig.esm.json", "src/index.ts"]) {
+    fs.mkdirSync(path.join(dir, path.dirname(file)), { recursive: true });
+    if (file.startsWith("src/")) fs.writeFileSync(path.join(dir, file), source);
+    else fs.copyFileSync(path.join(ROOT, file), path.join(dir, file));
+  }
+  for (const part of ["cjs", "esm", "types"]) {
+    fs.mkdirSync(path.join(dir, "dist", part), { recursive: true });
+    fs.writeFileSync(path.join(dir, "dist", part, "earlier.txt"), `${part} of the earlier build\n`);
+  }
+  fs.symlinkSync(path.join(ROOT, "node_modules"), path.join(dir, "node_modules"), "dir");
+  return dir;
+}
+
+// Every directory and file under dir with its content, node_modules left out.
+function snapshot(dir, prefix = "") {
+  const found = {};
+  for (const entry of fs.readdirSync(path.join(dir, prefix), { withFileTypes: true })) {
+    const name = path.join(prefix, entry.name);
+    if (name === "node_modules") continue;
+    if (!entry.isDirectory()) found[name] = fs.readFileSync(path.join(dir, name), "utf8");
+    else Object.assign(found, { [`${name}/`]: "" }, snapshot(dir, name));
+  }
+  return found;
+}
+
+function build(dir, ...args) {
+  return spawnSync(process.execPath, [path.join(dir, "scripts", "build.mjs"), ...args], { encoding: "utf8" });
+}
+
+test("a type error in src/: the build exits 1 and dist/ is as the earlier build left it", (t) => {
+  const dir = scratchPackage(t, "export const port: number = \"1883\";\n");
+  const before = snapshot(dir);
+
+  const run = build(dir);
+  assert.equal(run.status, 1, run.stdout + run.stderr);
+  assert.match(run.stdout, /error TS2322/);
+  assert.match(run.stderr, /^build: tsc -p tsconfig\.cjs\.json failed/m);
+  assert.deepEqual(snapshot(dir), before);
+});
+
+test("the second compiler run fails: what the first one wrote is dropped, dist/ is as the earlier build left it", (t) => {
+  // "export =" compiles as CommonJS and is refused in an ES module (TS1203)
+  const dir = scratchPackage(t, "const port = 1883;\nexport = port;\n");
+  const before = snapshot(dir);
+
+  const run = build(dir);
+  assert.equal(run.status, 1, run.stdout + run.stderr);
+  assert.match(run.stdout, /error TS1203/);
+  assert.match(run.stderr, /^build: tsc -p tsconfig\.esm\.json failed/m);
+  assert.deepEqual(snapshot(dir), before);
+});