Mocha Chai expect to throw not catching thrown errors

Question

I m having issues getting Chai s expect to throw to work in a test for my node js app  The test keeps failing on the thrown error  but If I wrap the test case in try and catch and assert on the caught error  it works    Does expect to throw not work like I think it should or something   it  should throw an error if you try to get an undefined property   function  done      var params     a   test   b   test   c   test       var model   new TestModel MOCK REQUEST  params         neither of these work   expect model get  z    to throw  Property does not exist in model schema       expect model get  z    to throw new Error  Property does not exist in model schema            this works   try        model get  z           catch err        expect err  to eql new Error  Property does not exist in model schema             done          The failure   19 passing  25ms    1 failing    1  Model Base should throw an error if you try to get an undefined property       Error  Property does not exist in model schema

User · Answer

This question has many  many duplicates  including questions not mentioning the Chai assertion library    Here are the basics collected together   The assertion must call the function  instead of it evaluating immediately   assert throws x y z               FAIL   x y z throws an exception  which immediately exits the       enclosing block  so assert throw   not called  assert throws     gt x y z           assert throw   is called with a function  which only throws       when assert throw executes the function  assert throws function      x y z              if you cannot use ES6 at work function badReference     x y z    assert throws badReference           for the verbose assert throws     gt model get z            the specific example given  homegrownAssertThrows model get  z          a style common in Python  but not in JavaScript   You can check for specific errors using any assertion library   Node    assert throws      gt  x y z     assert throws      gt  x y z  ReferenceError     assert throws      gt  x y z  ReferenceError   is not defined      assert throws      gt  x y z   is not defined      assert doesNotThrow      gt  42     assert throws      gt  x y z  Error     assert throws      gt  model get z   Property does not exist in model schema      Should    should throws      gt  x y z     should throws      gt  x y z  ReferenceError     should throws      gt  x y z  ReferenceError   is not defined      should throws      gt  x y z   is not defined      should doesNotThrow      gt  42     should throws      gt  x y z  Error     should throws      gt  model get z   Property does not exist in model schema      Chai Expect    expect      gt  x y z  to throw      expect      gt  x y z  to throw ReferenceError     expect      gt  x y z  to throw ReferenceError   is not defined      expect      gt  x y z  to throw  is not defined      expect      gt  42  not to throw      expect      gt  x y z  to throw Error     expect      gt  model get z  to throw  Property does not exist in model schema       You must handle exceptions that  escape  the test  it  should handle escaped errors   function        try       expect      gt  x y z  not to throw RangeError       catch  err        expect err  to be a ReferenceError             This can look confusing at first   Like riding a bike  it just  clicks  forever once it clicks

User · Answer

One other possible implementation  more cumbersome than the  bind   solution  but one that helps to make the point that expect   requires a function that provides a this context to the covered function  you can use a call    e g    expect function    model get call model   z      to throw

User · Answer

As this answer says  you can also just wrap your code in an anonymous function like this   expect function        model get  z       to throw  Property does not exist in model schema

User · Answer

examples from doc        because you rely on this context    which is lost when the function is invoked by  throw there   s no way for it to know what this is supposed to be   you have to use one of these options    wrap the method or function call inside of another function bind the context     wrap the method or function call inside of another function expect function      cat meow       to throw        Function expression expect      gt  cat meow    to throw                   ES6 arrow function     bind the context expect cat meow bind cat   to throw                 Bind

User · Answer

And if you are already using ES6 ES2015 then you can also use an arrow function  It is basically the same as using a normal anonymous function but shorter   expect      gt  model get  z    to throw  Property does not exist in model schema

User · Answer

I have found a nice way around it       The test  BDD style it   unsupported site        gt        The function myFunc       with arguments  url  https   www ebay com          should throw  unsupported             The function that does the magic   lang TypeScript  export const The          function    func Function    gt              with                   arguments   function     args any                    return      gt  func    args                                        It s much more readable then my old version     it   unsupported site        gt        const args    url  https   www ebay com       Arrange     function check unsupported site     myFunc args      Act     check unsupported site should throw  unsupported     Assert

User · Answer

You have to pass a function to expect  Like this   expect model get bind model   z    to throw  Property does not exist in model schema     expect model get bind model   z    to throw new Error  Property does not exist in model schema        The way you are doing it  you are passing to expect the result of calling model get  z    But to test whether something is thrown  you have to pass a function to expect  which expect will call itself  The bind method used above creates a new function which when called will call model get with this set to the value of model and the first argument set to  z    A good explanation of bind can be found here

[javascript] Mocha / Chai expect.to.throw not catching thrown errors

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