react-boilerplate/docs/js/reselect.md

# `reselect`

reselect memoizes ("caches") previous state trees and calculations based on said
tree. This means repeated changes and calculations are fast and efficient,
providing us with a performance boost over standard `mapStateToProps`
implementations.

The [official documentation](https://github.com/reactjs/reselect)
offers a good starting point!

## Usage

There are two different kinds of selectors, simple and complex ones.

### Simple selectors

Simple selectors are just that: they take the application state and select a
part of it.

```javascript
const mySelector = state => state.get('someState');

export { mySelector };
```

### Complex selectors

If we need to, we can combine simple selectors to build more complex ones which
get nested state parts with reselect's `createSelector` function. We import other
selectors and pass them to the `createSelector` call:

```javascript
import { createSelector } from 'reselect';
import mySelector from 'mySelector';

const myComplexSelector = createSelector(mySelector, myState =>
  myState.get('someNestedState'),
);

export { myComplexSelector };
```

These selectors can then either be used directly in our containers as
`mapStateToProps` functions or be nested with `createSelector` once again:

```javascript
export default connect(
  createSelector(myComplexSelector, myNestedState => ({ data: myNestedState })),
)(SomeComponent);
```

### Adding a new selector

If you have a `selectors.js` file next to the reducer which's part of the state
you want to select, add your selector to said file. If you don't have one yet,
add a new one into your container folder and fill it with this boilerplate code:

```JS
import { createSelector } from 'reselect';

const selectMyState = () => createSelector(

);

export {
  selectMyState,
};
```

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