How to test the type of a thrown exception in Jest

Question

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

Answer 1

I use a slightly more concise version:

expect(() => {
  // Code block that should throw error
}).toThrow(TypeError) // Or .toThrow('expectedErrorMessage')

Answer 2

The documentation is clear on how to do this. Let's say I have a function that takes two parameters and it will throw an error if one of them is null.

function concatStr(str1, str2) {
  const isStr1 = str1 === null
  const isStr2 = str2 === null
  if(isStr1 || isStr2) {
    throw "Parameters can't be null"
  }
  ... // Continue your code

Your test

describe("errors", () => {
  it("should error if any is null", () => {
    // Notice that the expect has a function that returns the function under test
    expect(() => concatStr(null, "test")).toThrow()
  })
})

Answer 3

Try:

expect(t).rejects.toThrow()

Answer 4

It is a little bit weird, but it works and IMHO is good readable:

it('should throw Error with message \'UNKNOWN ERROR\' when no parameters were passed', () => {
  try {
      throwError();
      // Fail test if above expression doesn't throw anything.
      expect(true).toBe(false);
  } catch (e) {
      expect(e.message).toBe("UNKNOWN ERROR");
  }
});

The Catch block catches your exception, and then you can test on your raised Error. Strange expect(true).toBe(false); is needed to fail your test if the expected Error will be not thrown. Otherwise, this line is never reachable (Error should be raised before them).

@Kenny Body suggested a better solution which improve a code quality if you use expect.assertions():

it('should throw Error with message \'UNKNOWN ERROR\' when no parameters were passed', () => {
  expect.assertions(1);
  try {
      throwError();
  } catch (e) {
      expect(e.message).toBe("UNKNOWN ERROR");
  }
});

See the original answer with more explanations: How to test the type of a thrown exception in Jest

Answer 5

In case you are working with Promises:

await expect(Promise.reject(new HttpException('Error message', 402)))
  .rejects.toThrowError(HttpException);

Answer 6

I haven't tried it myself, but I would suggest using Jest's toThrow assertion. So I guess your example would look something like this:

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

  expect(t).toThrowError('UNKNOWN ERROR');
  //or
  expect(t).toThrowError(TypeError);
});

Again, I haven't test it, but I think it should work.