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 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    gt      const error   t throws      gt        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

User · Answer

Modern Jest allows you to make more checks on a rejected value. For example:

const request = Promise.reject({statusCode: 404})
await expect(request).rejects.toMatchObject({ statusCode: 500 });

will fail with error

Error: expect(received).rejects.toMatchObject(expected)

- Expected
+ Received

  Object {
-   "statusCode": 500,
+   "statusCode": 404,
  }

User · Answer

From my  albeit limited  exposure to Jest  I have found that expect   toThrow   is suitable if you want to only test an error is thrown of a specific type  expect      gt  functionUnderTest    toThrow TypeError   Or an error is thrown with a specific message  expect      gt  functionUnderTest    toThrow  Something bad happened     If you try to do both  you will get a false positive  For example  if your code throws RangeError  Something bad happened     this test will pass  expect      gt  functionUnderTest    toThrow new TypeError  Something bad happened      The answer by bodolsog which suggests using a try catch is close  but rather than expecting true to be false to ensure the expect assertions in the catch are hit  you can instead use expect assertions 2  at the start of your test where 2 is the number of expected assertions  I feel this more accurately describes the intention of the test  A full example of testing the type and message of an error  describe  functionUnderTest        gt        it  should throw a specific type of error         gt            expect assertions 2            try               functionUnderTest              catch  error                expect error  toBeInstanceOf TypeError               expect error  toHaveProperty  message    Something bad happened                            If functionUnderTest   does not throw an error  the assertions will be be hit  but the expect assertions 2  will fail and the test will fail

User · Answer

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        gt      try         throwError             Fail test if above expression doesn t throw anything        expect true  toBe false       catch  e          expect e message  toBe  quot UNKNOWN ERROR quot             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        gt      expect assertions 1     try         throwError        catch  e          expect e message  toBe  quot UNKNOWN ERROR quot             See the original answer with more explanations  How to test the type of a thrown exception in Jest

User · Answer

Try  expect t  rejects toThrow

User · Answer

Further to Peter Danis  post  I just wanted to emphasize the part of his solution involving  quot  passing  a function into expect function  toThrow blank or type of error  quot   In Jest  when you test for a case where an error should be thrown  within your expect   wrapping of the function under testing  you need to provide one additional arrow function wrapping layer in order for it to work  I e  Wrong  but most people s logical approach   expect functionUnderTesting     toThrow ErrorTypeOrErrorMessage    Right  expect      gt    functionUnderTesting       toThrow ErrorTypeOrErrorMessage    It s very strange  but it should make the testing run successfully

User · Answer

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    gt      const error   t throws      gt        throwError         TypeError      expect t  toThrowError  UNKNOWN ERROR        or   expect t  toThrowError TypeError        Again  I haven t test it  but I think it should work

User · Answer

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  quot Parameters can t be null quot               Continue your code  Your test describe  quot errors quot        gt      it  quot should error if any is null quot        gt           Notice that the expect has a function that returns the function under test     expect      gt  concatStr null   quot test quot    toThrow

User · Answer

In case you are working with Promises   await expect Promise reject new HttpException  Error message   402       rejects toThrowError HttpException

User · Answer

Jest has a method  toThrow error   to test that a function throws when it is called  So  in your case you should call it so  expect t  toThrowError TypeError    The documentation

User · Answer

I ended up writing a convenience method for our test-utils library          Utility method to test for a specific error class and message in Jest     param  fn  expectedErrorClass  expectedErrorMessage       example   failTest         fn       gt            return new MyObject             param   stuff                            expectedErrorClass  MyError        expectedErrorMessage   stuff not yet implemented               failTest     fn  expectedErrorClass  expectedErrorMessage      gt        try         fn         expect true  toBeFalsy         catch  err          let isExpectedErr   err instanceof expectedErrorClass       expect isExpectedErr  toBeTruthy         expect err message  toBe expectedErrorMessage

User · Answer

In Jest you have to pass a function into expect function  toThrow  lt blank or type of error gt    Example  test  quot Test description quot        gt      const t        gt        throw new TypeError           expect t  toThrow TypeError        Or if you also want to check for error message  test  quot Test description quot        gt      const t        gt        throw new TypeError  quot UNKNOWN ERROR quot           expect t  toThrow TypeError     expect t  toThrow  quot UNKNOWN ERROR quot         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  quot Test description quot        gt      expect      gt   http get yourUrl  yourCallbackFn    toThrow TypeError

User · Answer

I use a slightly more concise version  expect      gt         Code block that should throw error    toThrow TypeError     Or  toThrow  expectedErrorMessage

[javascript] How to test the type of a thrown exception in Jest

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