How to validate thrown javascript exception using chai and mocha?

Question

I have MongoDB Query function which in which query params are validated Here is the function Note: user is mongoose model

function fetchData(uName)
{
    try{
        if(isParamValid(uName))
        {
            return user.find({"uName":uName}).exec()
        }
        else {
            throw "Invalid params"
        }
    }
    catch(e)
    {
        throw e
    }
}

To test this with invalid username values, I have written test code for that using mocha, chai, and chai-as-promised for promise-based functions

describe('Test function with invalid values', async ()=>{
    it('should catch exception', async () => {
        await expect(fetchData(inValidUserName)).to.throw()
    })

    it('should catch exception', async () => {
        await expect(fetchData(inValidUserName)).to.throw(Error)
    })

    it('should catch exception', async () => {
        await expect(fetchData(inValidUserName)).to.be.rejectedWith(Error)
    })

    it('should catch exception', async () => {
        await expect(fetchData(inValidUserName)).to.be.rejected
    })
})

None of them pass the test, How do I write a test case to handle exception for invalid userName values

Answer 1

You are passing the result of fetchData function call to expect function. Instead of calling the fetchData function inside expect function, pass a function to expect function.

it('should catch exception', async () => {
    await expect(() => fetchData(inValidUserName)).to.throw('Invalid params')
})

Answer 2

Use try/catch

it('should catch exception', async () => {
    try {
      await fetchData(inValidUserName);
    } catch(error) {
      expect(error).to.exist;
    }
})