How to test form submit with jest and enzyme in react?

Question

I am learning reactjs form with hooks, now I would like to test form on submit using jest and enzyme.

here is my login component.

import React from 'react'

function Login() {
    const [email, setEmail] = useState('');
    const [password, setPassword] = useState('');

    const handleSubmit = async (e) => {
        e.preventDefault();
        // ....api calLS
    }
    return (
        <div>
             <form onSubmit={handleSubmit} className="login">
    
            <input type="email" id="email-input" name="email" value={email} onChange={e => setEmail(e.target.value)} />
        
            <input type="password" id="password-input" name="password" value={password} onChange={e =>setPassword(e.target.value)} />
            
            <input type="submit" value="Submit" />
             </form> 
        </div>
    )
}

export default Login

Here is the login.test.js file

 describe('my sweet test', () => {
    it('clicks it', () => {
      
       const wrapper = shallow(<Login />);
       const updatedEmailInput = simulateChangeOnInput(wrapper, 'input#email-input', 'blah@gmail.com')
       const updatedPasswordInput = simulateChangeOnInput(wrapper, 'input#password-input', 'death'); 

       expect(updatedEmailInput.props().value).toEqual('blah@gmail.com');
       expect(updatedPasswordInput.props().value).toEqual('death');

       const instance = wrapper.instance()
       const spy = jest.spyOn(instance, 'handleSubmit')
   
       instance.forceUpdate();    
   
       const submitBtn = app.find('#sign-in')
       submitBtn.simulate('click')
       expect(spy).toHaveBeenCalled()

    })
    
  })

Unfortunately when I run npm test I get the following error.

What do I need to do to solve this error or can someone provide a tutorial on how to test a form submit?

Answer 1

In the documentation it's said that you cant use shallow.instance() for functional components It will return null: https://enzymejs.github.io/enzyme/docs/api/ShallowWrapper/instance.html There was also a previous answer on this topik Enzyme instance() returns null

You can pass validated function handleSubmit to Login as a prop like there How to use jest.spyOn with React function component using Typescript

 // Unit test
  describe('SomeComponent' () => {
  it('validates model on button click', () => {
      const handleSubmit = jest.fn();
      const wrapper = mount(
          <Login handleSubmit={handleSubmit}/>
      );
      const instance = wrapper.instance();
      const submitBtn = app.find('#sign-in')
      submitBtn.simulate('click')
      expect(handleSubmit).toHaveBeenCalled();
    });
  }

You need to call this test function handleSubmit in your login component either as a part of onSubmit or export whole onSubmit from upper components. Example login code with importing part of login function

import React from 'react'

function Login( {handleSubmit}) {
    const [email, setEmail] = useState('');
    const [password, setPassword] = useState('');

    const onSubmit = async (e) => {
        if (handleSubmit) {
          handleSubmit()
        }
        e.preventDefault();
        // ....api calLS
    }
    return (
        <div>
             <form onSubmit={onSubmit} className="login">
    
            <input type="email" id="email-input" name="email" value={email} onChange={e => setEmail(e.target.value)} />
        
            <input type="password" id="password-input" name="password" value={password} onChange={e =>setPassword(e.target.value)} />
            
            <input type="submit" value="Submit" />
             </form> 
        </div>
    )
}

export default Login

Example login code with importing of submit function

import React from 'react'

function Login( {handleSubmit}) {
    const [email, setEmail] = useState('');
    const [password, setPassword] = useState('');
   
    // handleSubmit is imported with props
 
    return (
        <div>
             <form onSubmit={handleSubmit} className="login">
    
            <input type="email" id="email-input" name="email" value={email} onChange={e => setEmail(e.target.value)} />
        
            <input type="password" id="password-input" name="password" value={password} onChange={e =>setPassword(e.target.value)} />
            
            <input type="submit" value="Submit" />
             </form> 
        </div>
    )
}

export default Login