test: initial test for onlyInstancesOf

Author: saintsebastian

__tests__/errors.test.js

@@ -0,0 +1,13 @@
+"use strict";
+
+const UsageError = require("../errors").UsageError;
+const onlyInstancesOf = require("../errors").onlyInstancesOf;
+
+describe("onlyInstancesOf", () => {
+  test("catches specified error", () => {
+    return Promise.reject(new UsageError('fake usage error'))
+      .catch(onlyInstancesOf(UsageError, (error) => {
+        expect(error).toBeInstanceOf(UsageError);
+      }));
+  });
+});