Javascript – How to test the type of a thrown exception in Jest

javascriptjestjsunit testing

I'm working with some code where I need to test the type of an exception thrown by a function (is it TypeError, ReferenceError, etc.?).

My current testing framework is AVA and I can test it as a second argument t.throws method, like here:

it('should throw Error with message \'UNKNOWN ERROR\' when no params were passed', (t) => {
  const error = t.throws(() => {
    throwError();
  }, TypeError);

  t.is(error.message, 'UNKNOWN ERROR');
});

I started rewriting my tests in Jest and couldn't find how to easily do that. Is it even possible?

Best Answer

In Jest you have to pass a function into expect(function).toThrow(<blank or type of error>).

Example:

test("Test description", () => {
  const t = () => {
    throw new TypeError();
  };
  expect(t).toThrow(TypeError);
});

Or if you also want to check for error message:

test("Test description", () => {
  const t = () => {
    throw new TypeError("UNKNOWN ERROR");
  };
  expect(t).toThrow(TypeError);
  expect(t).toThrow("UNKNOWN ERROR");
});

If you need to test an existing function whether it throws with a set of arguments, you have to wrap it inside an anonymous function in expect().

Example:

test("Test description", () => {
  expect(() => {http.get(yourUrl, yourCallbackFn)}).toThrow(TypeError);
});

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Python – How to test that a Python function throws an exception

Use TestCase.assertRaises (or TestCase.failUnlessRaises) from the unittest module, for example:

import mymod

class MyTestCase(unittest.TestCase):
    def test1(self):
        self.assertRaises(SomeCoolException, mymod.myfunc)

Best Answer

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Python – How to test that a Python function throws an exception

Related Topic