Mocking 一个 class 常数在开玩笑

Question

Say I have the below component that sets a constant, this.MAX_LENGTH in the constructor.

import PropTypes from 'prop-types';
import React from 'react';

class Input extends React.Component {
  static propTypes = {
    value: PropTypes.string.isRequired
  };

  constructor(props) {
    super(props);

    // An example constant
    this.MAX_LENGTH = 1024;
  }

  render() {
    
    return (
      <label htmlFor="comment_body">
        <textarea
          className="comment-reply input-highlight-on-focus"
          type="input"
          name="comment[body]"
          id="comment_body"
          maxLength={this.MAX_LENGTH}
          value={this.props.value} />
      </label>
    )
  }

}

export default Input;

The MAX_LENGTH constant is used to set the max length of the textarea .

In my Jest specs I want to mock the value of this.MAX_LENGTH , but I'm not sure how to set that mock.

Here's how my Jest test looks (it uses chai and enzyme as testing helpers):

it('renders the textarea', () => {
  // Mock the constant here to set a custom value of 99
  // ????

  const wrapper = mount(<Input value={'foo'} />)
  const textarea = wrapper.find('.comment-reply').first();

  expect(textarea).to.have.attr('maxLength', '99');
});

What can I replace ???? with mock the value of the constant?

I tried reading through ES6 Class Mocks in the Jest docs but it seemed to be around mocking a whole imported class and I'm not sure how it would apply to a single constant.

Thanks!

Answer 1

Using instance properties for constants is considered a bad practice; this is what static properties are for. This would be possible to mock as Input.MAX_LENGTH =... before mounting the component:

class Input extends React.Component {
  static MAX_LENGTH = 1024;
  ...
}

Original value needs to be restored in afterEach .

Or at least making it read-only prototype property. This would be possible to mock as jest.spyOn(Input, 'MAX_LENGTH', 'get').mockReturnValue(...) before mounting the component:

class Input extends React.Component {
  get MAX_LENGTH() { return 1024 };
  ...
}

Without that, it needs to be mocked on component instance after initial render:

const wrapper = mount(<Input value={'foo'} />)
wrapper.instance().MAX_LENGTH = ...;
wrapper.setProps({});
...