validate parallel config

Author: davidtaylorhq

packages/babel/src/index.js

@@ -151,6 +151,12 @@ function createBabelInputPluginFactory(customCallback = returnObject) {
       typeof customFilter === 'function' ? customFilter : createFilter(include, exclude);
     const filter = (id, code) => extensionRegExp.test(id) && userDefinedFilter(id, code);
 
+    if (typeof parallel === 'number' && (!Number.isInteger(parallel) || parallel < 1)) {
+      throw new Error(
+        'The "parallel" option must be true or a positive integer specifying the number of workers.'
+      );
+    }
+
     if (parallel) {
       const parallelAllowed =
         isSerializable(babelOptions) && !overrides?.config && !overrides?.result;

packages/babel/test/as-input-plugin.mjs

@@ -611,6 +611,21 @@ test('works in parallel with specified worker count', async () => {
   expect(code.includes('var answer = 42')).toBe(true);
 });
 
+test('throws when parallel option is not a positive integer', () => {
+  expect(() => babelPlugin({ babelHelpers: 'bundled', parallel: 0 })).toThrow(
+    /must be true or a positive integer/
+  );
+  expect(() => babelPlugin({ babelHelpers: 'bundled', parallel: -1 })).toThrow(
+    /must be true or a positive integer/
+  );
+  expect(() => babelPlugin({ babelHelpers: 'bundled', parallel: 2.5 })).toThrow(
+    /must be true or a positive integer/
+  );
+  expect(() => babelPlugin({ babelHelpers: 'bundled', parallel: NaN })).toThrow(
+    /must be true or a positive integer/
+  );
+});
+
 test('throws when using parallel with non-serializable babel options', async () => {
   await expect(() =>
     generate('basic/main.js', {