React js - How to mock Context when testing component

Question

I'm trying to test a component which inherits context from a root component, without loading/rendering everything from the root down. I've tried and searched for examples on how to mock the context but can't find anything (at least that doesn't use jest).

Here's a simplified example of what I'm trying to achieve.

Is there a simple way I can mock reactEl.context for the test?

/**
* Root Element that sets up & shares context
*/
class Root extends Component {
  getChildContext() {
    return { 
      language: { text: 'A String'} 
    };
  }

  render() {
    return (
      <div>
        <ElWithContext />
      </div>
    );
  }
}

Root.childContextTypes = { language: React.PropTypes.object };

/**
 * Child Element which uses context
 */
class ElWithContext extends React.Component{
  render() {
    const {language} = this.context;
    return <p>{language.text}</p>
  }
}

ElWithContext.contextTypes = { language: React.PropTypes.object }



/**
 * Example test where context is unavailable.
 */
let el = React.createElement(ElWithContext)

element = TestUtils.renderIntoDocument(el);
// ERROR: undefined is not an object (evaluating 'language.text')

describe("ElWithContext", () => {
  it('should contain textContent from context', () => {
    const node = ReactDOM.findDOMNode(element);
    expect(node.textContent).to.equal('A String');
  });
})

Answer 1

I went into the same issue as you did and found out two ways of doing it.

The first one is a basic copycat of your own way: Create a wrapper around my component and inject it with a dynamic context. I put the source code below for those interested, because it's ES6 unlike your example. But it's just to show how it would be done in ES6 and I do NOT recommend anyone using it (I haven't actually tested it myself).

src/testUtils/mockWithContext.js

import React, { Component } from 'react';
import wrapDisplayName from 'recompose/wrapDisplayName';
import hoistStatics from 'recompose/hoistStatics';

export const defaultContext = {
  permissions: [

  ],
  user: {
    id: '1',
    display_name: 'Default user',
    email: '<your.email>+default.user@gmail.com', // Trick with "+" for infinite aliases using gmail.
    username: 'default_user',
    created: '2016-08-01T15:50:13.246Z',
  },
};

export const defaultContextType = {
  permissions: React.PropTypes.array,
  user: React.PropTypes.shape({
    id: React.PropTypes.string.isRequired,
    display_name: React.PropTypes.string.isRequired,
    email: React.PropTypes.string.isRequired,
    username: React.PropTypes.string.isRequired,
    created: React.PropTypes.string.isRequired,
  }),
};

/**
 * HOC for context
 */
const withContext = ({ context = defaultContext, contextType = defaultContextType }) => (WrappedComponent) => {
  class WithContext extends Component {
    getChildContext() {
      return context;
    }

    render() {
      return <WrappedComponent {...this.props} />;
    }
  }

  WithContext.displayName = wrapDisplayName(WrappedComponent, 'WithContext');
  WithContext.WrappedComponent = WrappedComponent;
  WithContext.childContextTypes = contextType;

  return WithContext;
};

export default hoistStatics(withContext);

---

As I said, I wrote it, but didn't test it because I found a much better way of doing context-injecting when trying to write tests for this mock.

Using Enzyme library, which is definitely built to support React components testing, there is the ability to shallow/mount/static render your component, for testing purpose. And each of these methods allow a second argument: the context.

SimpleComponent.js

const SimpleComponent = React.createClass({
  contextTypes: {
    name: React.PropTypes.string,
  },
  render() {
    return <div>{this.context.name}</div>;
  },
});

SimpleComponent.test.js

const context = { name: 'foo' };
const wrapper = mount(<SimpleComponent />, { context });
expect(wrapper.text()).to.equal('foo');
wrapper.setContext({ name: 'bar' });
expect(wrapper.text()).to.equal('bar');
wrapper.setContext({ name: 'baz' });
expect(wrapper.text()).to.equal('baz');

Pretty straight-forward. I didn't use it yet but it looks like what I (and you) wanted to do. I guess I'll just have to throw my own implementation to the garbage.

http://airbnb.io/enzyme/docs/api/ReactWrapper/setContext.html

Answer 2

I went with a solution of creating a wrapping component with context. Not sure if this is a great approach but is working for me right now:

/**
* Helper function to wrap component with a component that has context 
*/
function wrapWithContext(context, contextTypes, children, React){

    const wrapperWithContext = React.createClass({
        childContextTypes: contextTypes,
        getChildContext: function() { return context },
        render: function() { return React.createElement('div', null, children) }
    });

  return React.createElement(wrapperWithContext);
}

/**
* Usage
*/

// in setup function of test framework
const el = React.createElement(ElWithContext);

const context = { language: { text: 'A String' } };
const contextTypes = { language: React.PropTypes.object };
const wrapper = wrapWithContext(context, contextTypes, [el], React);
const ElWithContext = TestUtils.renderIntoDocument(wrapper);

// do tests
describe('ElWithContext', () => {
   it('should contain textContent from context', () => {
      const node = ReactDOM.findDOMNode(element);
      expect(node.textContent).to.equal('A String');
   });
})