feat(react): add warning helper

packages/react/src/internal/__tests__/warning-test.js

@@ -0,0 +1,42 @@
+/**
+ * Copyright IBM Corp. 2016, 2018
+ *
+ * This source code is licensed under the Apache-2.0 license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import { warning } from '../warning';
+
+describe('warning', () => {
+  test('calls console.warn() when the condition is false', () => {
+    const spy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+    warning(false, 'The message');
+
+    expect(spy).toHaveBeenCalledWith('Warning: The message');
+    spy.mockRestore();
+  });
+
+  test('does not call console.warn() when the condition is true', () => {
+    const spy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+
+    warning(true, 'The message');
+
+    expect(spy).not.toHaveBeenCalled();
+    spy.mockRestore();
+  });
+
+  test('throws an error when no message is provided', () => {
+    expect(() => {
+      warning(true);
+    }).toThrow();
+  });
+
+  test('substitutes extra arguments in the message', () => {
+    const spy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+
+    warning(false, '%s %s %s', 'a', 'b', 'c');
+
+    expect(spy).toHaveBeenCalledWith('Warning: a b c');
+    spy.mockRestore();
+  });
+});

packages/react/src/internal/warning.js

@@ -0,0 +1,29 @@
+/**
+ * Copyright IBM Corp. 2016, 2018
+ *
+ * This source code is licensed under the Apache-2.0 license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const emptyFunction = function () {};
+
+const warning = __DEV__
+  ? function warning(condition, format, ...args) {
+      if (format === undefined) {
+        throw new Error(
+          '`warning(condition, format, ...args)` requires a warning ' +
+            'format argument'
+        );
+      }
+      if (!condition) {
+        let index = 0;
+        const message = format.replace(/%s/g, () => {
+          return args[index++];
+        });
+
+        console.warn('Warning: ' + message);
+      }
+    }
+  : emptyFunction;
+
+export { warning };