Jest onSpy does not recognize React component function

Question

This one has me totally stumped despite trying everything.

I am testing a React component with Jest/Enzyme. This test simulates modifying a element which then calls an onChange method.

When I run test I get this from Jest:

Cannot spy the onChange property because it is not a function; undefined given instead

Why??

Here are the key parts of the component:

import React, {Component} from 'react';
import EntitiesPulldown from './entities-pulldown'

class NewTransactionForm extends Component  {

constructor(props) {
   super(props);

   this.state = {
       amount: "0.00",
       investors_4_picklist: [],
       selectedTType:0      
   };

   this.onChange = this.onChange.bind(this);
} 

onChange(event) {
      const value = event.target.value;
      const name = event.target.name;

      this.setState({
        [name]: value
      });
      console.log("just set "+name+"  to  "+value)

}

render() {
return (  
      <div>
           <EntitiesPulldown                                                                           
               itemList = {this.state.investors_4_picklist}                                                                                                                                                      
               handleChangeCB = {this.onChange}
             />
      </div>
  )
 }
}
export default NewTransactionForm;

Pretty straightforward. And here is the test:

test('NTF calls onChange after select', () => {
  const wrapperInstance = mount(<NewTransactionForm />).instance();

  const spy = jest.spyOn(wrapperInstance, 'onChange')  //fails

  wrapperInstance.update().find('select').simulate('change',{target: { name: 'selectedTType', value : 3}});

  expect(spy).toHaveBeenCalled();
});

And I also tried this option, same results:

  const spy = jest.spyOn(NewTransactionForm.prototype, 'onChange')

What am I doing wrong? Any help really appreciated.

As far as I can tell, that looks correct. I would check assumptions to make sure wrapperInstance is the object you expect it to be, and that you aren't doing something like accidentally importing the wrong component... — Garrett Motzner
If you can include the header/imports for the test file, that might help. — Garrett Motzner
What about spying on the prototype? jest.spyOn(wrapperInstance.prototype, 'onChange') — evolutionxbox

Paul Kogan Paul Kogan · Accepted Answer · 2018-10-12T16:51:29

Thanks Garret. I have a new view into the problem, but not the answer. The component that was deployed was a wrapped HOC in order to use Material-UI Styles. Because it was an HOC, it was losing access to all the methods of the wrapped component.

The actual export was:

NewTransactionForm.propTypes = {
  classes: PropTypes.object.isRequired,
};

export default withStyles(styles)(NewTransactionForm);

When I took out the styling and exported with:

export default NewTransactionForm;

I was able to access the onChange function. I am now looking for ways to make the methods of the wrapped component accessible to outside calls, but that is another question.

Jest onSpy does not recognize React component function

1 Answers