Add unit tests

packages/material-ui/src/utils/useId.test.js

@@ -0,0 +1,30 @@
+import * as React from 'react';
+import PropTypes from 'prop-types';
+import { expect } from 'chai';
+import { createClientRender } from 'test/utils/createClientRender';
+import useId from './useId';
+
+const TestComponent = ({ id: idProp }) => {
+  const id = useId(idProp);
+  return <span>{id}</span>;
+};
+
+TestComponent.propTypes = {
+  id: PropTypes.string,
+};
+
+describe('useId', () => {
+  const render = createClientRender();
+
+  it('returns the provided ID', () => {
+    const { getByText } = render(<TestComponent id="some-id" />);
+
+    expect(getByText('some-id')).to.not.be.null;
+  });
+
+  it("generates an ID if one isn't provided", () => {
+    const { getByText } = render(<TestComponent />);
+
+    expect(getByText(/^mui-[0-9]+$/)).to.not.be.null;
+  });
+});