Testing onChange function in Jest

Syntax on your code snippet I think should be:

import React from 'react';

export default class InputBox extends React.Component {
  onSearch(event) {
    event.preventDefault();
    this.props.onSearch(event.target.value.trim());
  }
  render () { return (<input onChange={this.onSearch.bind(this)} />); }
}

The test is failing because, as same you define the preventDefault function on the event object, you also must define other properties used on the onSearch function.

it('should call onChange prop', () => {
  const onSearchMock = jest.fn();
  const event = {
    preventDefault() {},
    target: { value: 'the-value' }
  };
  const component = enzyme.shallow(<InputBox onSearch={onSearchMock} />);
  component.find('input').simulate('change', event);
  expect(onSearchMock).toBeCalledWith('the-value');
});

Previous test code needs to define the event shape because you are using shallow rendering. If you want instead to test that the actual input value is being used on your onSearch function you need to try a full render with enzyme.mount:

it('should call onChange prop with input value', () => {
  const onSearchMock = jest.fn();
  const component = enzyme.mount(<InputBox onSearch={onSearchMock} value="custom value" />);
  component.find('input').simulate('change');
  expect(onSearchMock).toBeCalledWith('custom value');
});