rereselect

> Not to be confused with
> Re-reselect which is an
> enhancement to Reselect. This is an
> entirely separate project.

A library that generates memoized selectors like
Reselect but:

- Supports dynamic dependency tracking à la Vue/VueX/MobX. See
my StackOverflow answer for the
motivation why we need this.
- No need to declare upfront which selectors will be used.
- Introspection (hooks) API baked in to help debug performance problems.

Design constraints:

- Generated selector must be compatible with Reselect.

Notes:

- Requires an ES6 environment (or babel-polyfill).
- TypeScript typings require TypeScript 3.0.
- The state must be immutable.
- The selector logic must be pure and deterministic.
- rereselect’s selectors take 1 argument only — the state. If you need
parameterized selectors, see the section
parameterized selectors.
- No support. This library is created to solve the problems we face. We
open-source it in hope that it will be useful to others as well, but we have
no plans in supporting it beyond our use cases. Therefore, feature requests
are not accepted here.

Differences from Reselect?

The Reselect “shopping cart” example:

``js
import { makeSelector } from '@taskworld.com/rereselect'

// “Simple” selectors are the same.
const shopItemsSelector = state => state.shop.items
const taxPercentSelector = state => state.shop.taxPercent

// Instead of createSelector, it is called makeSelector.
//
// Instead of declaring dependencies upfront, use the query function
// to invoke other selectors. In doing so, the dependency will
// automatically be tracked.
//
const subtotalSelector = makeSelector(query =>
query(shopItemsSelector).reduce((acc, item) => acc + item.value, 0)
)
const taxSelector = makeSelector(
query => query(subtotalSelector) * (query(taxPercentSelector) / 100)
)
const totalSelector = makeSelector(query => ({
total: query(subtotalSelector) + query(taxSelector)
}))
`

Dynamic dependency tracking:

`js
let state = {
fruits: {
a: { name: 'Apple' },
b: { name: 'Banana' },
c: { name: 'Cantaloupe' }
},
selectedFruitIds: ['a', 'c']
}

// I want to query the selected fruits...
const selectSelectedFruits = makeSelector(query =>
query(state => state.selectedFruitIds).map(id =>
query(state => state.fruits[id])
)
)

// Use like any other selectors:
console.log(selectSelectedFruits(state)) // [ { name: 'Apple' }, { name: 'Cantaloupe' } ]

// Since data selection is fine-grained, changes to unrelated parts
// of the state will not cause a recomputation.
state = {
...state,
fruits: {
...state.fruits,
b: { name: 'Blueberry' }
}
}
console.log(selectSelectedFruits(state)) // [ { name: 'Apple' }, { name: 'Cantaloupe' } ]
console.log(selectSelectedFruits.recomputations()) // 1
`

Parameterized selectors

This library is only concerned with creating a selector system that supports
dynamic dependency tracking. So, it is up to you to implement parameterized
selectors support.

This is how we do it (we also added displayName property to our selectors to
make them easier to debug):

`typescript
export function makeParameterizedSelector(
displayName,
selectionLogicGenerator
) {
const memoized = new Map()
return Object.assign(
function selectorFactory(...args) {
const key = args.join(',')
if (memoized.has(key)) return memoized.get(key)!
const name = ${displayName}(${key})
const selectionLogic = selectionLogicGenerator(...args)
const selector = makeSelector(selectionLogic)
selector.displayName = name
memoized.set(key, selector)
return selector
},
{ displayName }
)
}
``