diff --git a/scripts/build.mjs b/scripts/build.mjs
index 3124b54..e170965 100644
--- a/scripts/build.mjs
+++ b/scripts/build.mjs
@@ -14,42 +14,59 @@ import { createRequire } from "node:module";
 import { basename, dirname, join, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 
+const USAGE = "usage: node scripts/build.mjs [--out <dir>]";
 const root = resolve(dirname(fileURLToPath(import.meta.url)), "..");
-const tsc = createRequire(import.meta.url).resolve("typescript/bin/tsc");
 
 function outputDirectory(args) {
-  const at = args.indexOf("--out");
-  if (at === -1) return join(root, "dist");
-  if (!args[at + 1]) throw new Error("--out needs a directory");
-  return resolve(args[at + 1]);
+  if (args.length === 0) return join(root, "dist");
+  const [flag, dir, ...rest] = args;
+  // A misspelt --out must not fall back to the default and replace the tracked dist/
+  if (flag !== "--out") throw new Error(`unknown argument ${flag}; ${USAGE}`);
+  if (!dir) throw new Error(`--out needs a directory; ${USAGE}`);
+  if (rest.length > 0) throw new Error(`unknown argument ${rest[0]}; ${USAGE}`);
+  return resolve(dir);
 }
 
-function compile(project, options) {
-  const run = spawnSync(process.execPath, [tsc, "-p", join(root, project), ...options], { cwd: root, stdio: "inherit" });
-  if (run.error) throw run.error;
-  if (run.status !== 0) throw new Error(`tsc -p ${project} failed, ${out} is unchanged`);
+function compiler() {
+  try {
+    return createRequire(import.meta.url).resolve("typescript/bin/tsc");
+  } catch (err) {
+    if (err.code !== "MODULE_NOT_FOUND") throw err;
+    throw new Error("typescript is not installed: run npm install (it is a devDependency)");
+  }
 }
 
-let out;
-let staging;
-try {
-  out = outputDirectory(process.argv.slice(2));
+function build(args) {
+  const out = outputDirectory(args);
+  const tsc = compiler();
   // Beside the output, not in it: the package ships all of dist/, and a build that is killed cannot remove its
   // staging directory - the next build does. Same parent directory, so the renames below stay on one filesystem.
-  staging = join(dirname(out), `.${basename(out)}-staging`);
+  const staging = join(dirname(out), `.${basename(out)}-staging`);
+  const compile = (project, options) => {
+    const run = spawnSync(process.execPath, [tsc, "-p", join(root, project), ...options], { cwd: root, stdio: "inherit" });
+    if (run.error) throw run.error;
+    if (run.status !== 0) throw new Error(`tsc -p ${project} failed, ${out} is unchanged`);
+  };
+
   rmSync(staging, { recursive: true, force: true });
-  compile("tsconfig.cjs.json", ["--outDir", join(staging, "cjs"), "--declarationDir", join(staging, "types")]);
-  compile("tsconfig.esm.json", ["--outDir", join(staging, "esm")]);
-  // The package is "type": "commonjs"; this marks the .js files under esm/ as ES modules.
-  writeFileSync(join(staging, "esm", "package.json"), JSON.stringify({ type: "module" }, null, 2) + "\n");
-  mkdirSync(out, { recursive: true });
-  for (const part of ["cjs", "esm", "types"]) {
-    rmSync(join(out, part), { recursive: true, force: true });
-    renameSync(join(staging, part), join(out, part));
+  try {
+    compile("tsconfig.cjs.json", ["--outDir", join(staging, "cjs"), "--declarationDir", join(staging, "types")]);
+    compile("tsconfig.esm.json", ["--outDir", join(staging, "esm")]);
+    // The package is "type": "commonjs"; this marks the .js files under esm/ as ES modules.
+    writeFileSync(join(staging, "esm", "package.json"), JSON.stringify({ type: "module" }, null, 2) + "\n");
+    mkdirSync(out, { recursive: true });
+    for (const part of ["cjs", "esm", "types"]) {
+      rmSync(join(out, part), { recursive: true, force: true });
+      renameSync(join(staging, part), join(out, part));
+    }
+  } finally {
+    rmSync(staging, { recursive: true, force: true });
   }
+}
+
+try {
+  build(process.argv.slice(2));
 } catch (err) {
   console.error(`build: ${err.message}`);
   process.exitCode = 1;
-} finally {
-  if (staging) rmSync(staging, { recursive: true, force: true });
 }
diff --git a/test/build.test.js b/test/build.test.js
index 4df325e..281f6f8 100644
--- a/test/build.test.js
+++ b/test/build.test.js
@@ -11,8 +11,8 @@ 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) {
+// build; removed when the test ends. installed: false leaves node_modules out, as before npm install.
+function scratchPackage(t, source, { installed = true } = {}) {
   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"]) {
@@ -24,7 +24,7 @@ function scratchPackage(t, source) {
     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");
+  if (installed) fs.symlinkSync(path.join(ROOT, "node_modules"), path.join(dir, "node_modules"), "dir");
   return dir;
 }
 
@@ -82,3 +82,30 @@ test("the second compiler run fails: what the first one wrote is dropped, dist/
   assert.match(run.stderr, /^build: tsc -p tsconfig\.esm\.json failed/m);
   assert.deepEqual(snapshot(dir), before);
 });
+
+test("an argument the script does not know is refused: a misspelt --out must not build into dist/", (t) => {
+  const dir = scratchPackage(t, "export const port = 1883;\n");
+  const before = snapshot(dir);
+  const elsewhere = path.join(dir, "elsewhere");
+  const refused = [
+    [["--outt", elsewhere], "unknown argument --outt"],
+    [["--out"], "--out needs a directory"],
+    [["--out", elsewhere, "--watch"], "unknown argument --watch"],
+  ];
+  for (const [args, reason] of refused) {
+    const run = build(dir, ...args);
+    assert.equal(run.status, 1, args.join(" "));
+    assert.equal(run.stderr, `build: ${reason}; usage: node scripts/build.mjs [--out <dir>]\n`);
+    assert.deepEqual(snapshot(dir), before, args.join(" "));
+  }
+});
+
+test("without typescript the build says what to install, in one line", (t) => {
+  const dir = scratchPackage(t, "export const port = 1883;\n", { installed: false });
+  const before = snapshot(dir);
+
+  const run = build(dir);
+  assert.equal(run.status, 1);
+  assert.equal(run.stderr, "build: typescript is not installed: run npm install (it is a devDependency)\n");
+  assert.deepEqual(snapshot(dir), before);
+});