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. Read the motivation section to see 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.

npm install @taskworld.com/rereselect

Why a new selector library?

Here’s an example. Let’s say we have a list of online user IDs and a mapping from user ID to user’s information. We want to select a list of online users, as in this example:

Creating such selector is impossible using Reselect, because in Reselect, selectors must declare their dependencies statically upfront. Since we couldn’t know in advance which users will be online, we need to declare a dependency on the whole users branch of the state tree:

This results in a selector that looks like this:

const selectOnlineUsers = createSelector(
  state => state.onlineUserIds,
  state => state.users,
  (onlineUserIds, users) => {
    return onlineUserIds.map(id => users[id])
  }
)

This works, but this means that changes to unrelated users (bob, charlie, eve) will cause the selector to be recomputed. This problem has been asked multiple times with no efficient and elegant solution.

With rereselect, selectors don’t declare their dependencies upfront. Instead, they are inlined in the selection logic:

const selectOnlineUsers = makeSelector(query => {
  const userIds = query(state => state.onlineUserIds)
  return userIds.map(id => query(state => state.users[id]))
})

The selection logic will receive a function query which can be used to invoke other selectors. In doing so, the dependency will be tracked automatically. This allows more fine-grained control over which part of the state tree would cause the selector to be recomputed.

The Reselect “shopping cart” example:

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:

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

Reimplementing Reselect’s createSelector on top of rereselect:

function createSelector(...funcs) {
  const resultFunc = funcs.pop()
  const dependencies = Array.isArray(funcs[0]) ? funcs[0] : funcs
  return makeSelector(query => resultFunc(...dependencies.map(query)))
}

As of v0.3.0, in “cache hit” scenarios, rereselect is faster than Reselect in ES6 environment.

Performance benchmarks are run as part of CI build. You can check out the latest results there.

This library is only concerned with creating a selector system that supports dynamic dependency tracking. It provides a building blocks for which higher-level abstractions can be built upon. So, it is up to you to built your tooling on top of this.

Please read the test to see some of the real-world usage scenarios.

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

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 }
  )
}

Please read the test.