# Motivation

## Why does this library exist?

React components appear incredibly simple to test at first glance. After all, they're just functions of their state and props, right?

```javascript
function MyComponent(props) {
  return (
    <div>
      I have worked for {props.company} for {props.years} years.
    </div>
  );
}

const wrapper = mount(<MyComponent company="TrialSpark" years={2} />);
expect(wrapper.text()).toBe('I have worked at TrialSpark for 2 years.');

wrapper.setProps({ company: 'Facebook', years: 0 });
expect(wrapper.text()).toBe('I have worked at Facebook for 0 years.');
```

**Wrong!** And there's a *single* react feature we can blame for this: context. Context allows components to provide data that *all* its ancestors access *without* passing that data down as props. In practice, this is a helpful feature, and it powers some of the most popular libraries out there today: `react-redux`, `react-router`, `react-apollo`, to just name a few.

However, it creates some real chalenges when we go to test our components:

```javascript
let MyComponent = function MyComponent(props) {
  return <div>My name is {props.name}.</div>;
};
const mapStateToProps = state => ({
  name: state.user.name,
});
MyComponent = connect(mapStateToProps)(MyComponent);

// Error: store missing in context
const wrapper = mount(<MyComponent />);
```

One common way to solve this problem is to render your component inside of a provider:

```javascript
import { Provider } from 'react-redux';
import store from './myStore';

const wrapper = mount(
  <Provider store={store}>
    <MyComponent />
  </Provider>,
);
```

However, this doesn't play very nicely with enzyme:

```javascript
wrapper.type(); // Provider, not MyComponent
wrapper.setProps({}); // Setting props on the Provider, not MyComponent
wrapper.instance(); // Instance of Provider, not MyComponent
```

Thankfully, `enzyme` allows us to pass context to our components when we mount this:

```javascript
import PropTypes from 'prop-types';
import { Provider } from 'react-redux';
import store from './myStore';

const wrapper = mount(<MyComponent />, {
  context: {
    store,
  },
  childContextTypes: {
    store: PropTypes.any,
  },
});

wrapper.type(); // MyComponent
```

## The problems with passing context to enzyme

While the above solution works, its imperfect for a couple of reasons:

### 1) Context is global to the app, but we have to pass it in *every test*

Contexts are usually global to our app. For example, an app typically has one store, one router, etc. However, passing context to enzyme requires us to setup our global context *every time* we mount a component. This leads to a lot of boilerplate when writing tests, and allows for the possibility of not bootstrapping context correctly, making our tests potentially less effective. This becomes more of a problem as we add more libraries that use context to our app.

### 2) More importantly, passing context to enzyme requires us to know about private APIs

Libraries like `react-redux` do a good job of shielding us from context. It's API consists entirely of components: just wrap your whole app in a `<Provider />` and you're off to the races! However, to pass context to enzyme correctly, we need to know about the *implementation* of the `react-redux` provider—what context does it provide to its children?

`React-redux` is pretty simple. It just passes a `store` as context. Other libraries are more complicated. And, like before, the more libraries we add, the more internals we must know about.

## How does enzyme-context solve this?

Enzyme Context solves this problem by allowing you to *create* a custom `mount`/`shallow` function for your app that has all of your app's context set up for you already! It is built on top of a plugin system so you can add as many plugins as are necessary for the libraries in your app. This way, the *plugins* can worry about the interals of the libraries they are supporting, and you can focus on testing your application.
