Name: reselect
Owner: SalesforceIQ
Description: Selector library for Redux
Created: 2017-07-25 20:52:28.0
Updated: 2018-04-26 20:32:02.0
Pushed: 2017-08-15 17:47:51.0
Homepage: null
Size: 397
Language: JavaScript
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Simple ?selector? library for Redux inspired by getters in NuclearJS, subscriptions in re-frame and this proposal from speedskater.
rt { createSelector } from 'reselect'
t shopItemsSelector = state => state.shop.items
t taxPercentSelector = state => state.shop.taxPercent
t subtotalSelector = createSelector(
opItemsSelector,
ems => items.reduce((acc, item) => acc + item.value, 0)
t taxSelector = createSelector(
btotalSelector,
xPercentSelector,
ubtotal, taxPercent) => subtotal * (taxPercent / 100)
rt const totalSelector = createSelector(
btotalSelector,
xSelector,
ubtotal, tax) => ({ total: subtotal + tax })
exampleState = {
op: {
taxPercent: 8,
items: [
{ name: 'apple', value: 1.20 },
{ name: 'orange', value: 0.95 },
]
ole.log(subtotalSelector(exampleState)) // 2.15
ole.log(taxSelector(exampleState)) // 0.172
ole.log(totalSelector(exampleState)) // { total: 2.322 }
Why isn't my selector recomputing when the input state changes?
Why is my selector recomputing when the input state stays the same?
The default memoization function is no good, can I use a different one?
npm install reselect
The examples in this section are based on the Redux Todos List example.
containers/VisibleTodoList.jsrt { connect } from 'react-redux'
rt { toggleTodo } from '../actions'
rt TodoList from '../components/TodoList'
t getVisibleTodos = (todos, filter) => {
itch (filter) {
case 'SHOW_ALL':
return todos
case 'SHOW_COMPLETED':
return todos.filter(t => t.completed)
case 'SHOW_ACTIVE':
return todos.filter(t => !t.completed)
t mapStateToProps = (state) => {
turn {
todos: getVisibleTodos(state.todos, state.visibilityFilter)
t mapDispatchToProps = (dispatch) => {
turn {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
t VisibleTodoList = connect(
pStateToProps,
pDispatchToProps
doList)
rt default VisibleTodoList
In the above example, mapStateToProps calls getVisibleTodos to calculate todos. This works great, but there is a drawback: todos is calculated every time the component is updated. If the state tree is large, or the calculation expensive, repeating the calculation on every update may cause performance problems. Reselect can help to avoid these unnecessary recalculations.
We would like to replace getVisibleTodos with a memoized selector that recalculates todos when the value of state.todos or state.visibilityFilter changes, but not when changes occur in other (unrelated) parts of the state tree.
Reselect provides a function createSelector for creating memoized selectors. createSelector takes an array of input-selectors and a transform function as its arguments. If the Redux state tree is mutated in a way that causes the value of an input-selector to change, the selector will call its transform function with the values of the input-selectors as arguments and return the result. If the values of the input-selectors are the same as the previous call to the selector, it will return the previously computed value instead of calling the transform function.
Let's define a memoized selector named getVisibleTodos to replace the non-memoized version above:
selectors/index.jsrt { createSelector } from 'reselect'
t getVisibilityFilter = (state) => state.visibilityFilter
t getTodos = (state) => state.todos
rt const getVisibleTodos = createSelector(
getVisibilityFilter, getTodos ],
isibilityFilter, todos) => {
switch (visibilityFilter) {
case 'SHOW_ALL':
return todos
case 'SHOW_COMPLETED':
return todos.filter(t => t.completed)
case 'SHOW_ACTIVE':
return todos.filter(t => !t.completed)
}
In the example above, getVisibilityFilter and getTodos are input-selectors. They are created as ordinary non-memoized selector functions because they do not transform the data they select. getVisibleTodos on the other hand is a memoized selector. It takes getVisibilityFilter and getTodos as input-selectors, and a transform function that calculates the filtered todos list.
A memoized selector can itself be an input-selector to another memoized selector. Here is getVisibleTodos being used as an input-selector to a selector that further filters the todos by keyword:
t getKeyword = (state) => state.keyword
t getVisibleTodosFilteredByKeyword = createSelector(
getVisibleTodos, getKeyword ],
isibleTodos, keyword) => visibleTodos.filter(
todo => todo.text.includes(keyword)
If you are using React Redux, you can call selectors as regular functions inside mapStateToProps():
containers/VisibleTodoList.jsrt { connect } from 'react-redux'
rt { toggleTodo } from '../actions'
rt TodoList from '../components/TodoList'
rt { getVisibleTodos } from '../selectors'
t mapStateToProps = (state) => {
turn {
todos: getVisibleTodos(state)
t mapDispatchToProps = (dispatch) => {
turn {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
t VisibleTodoList = connect(
pStateToProps,
pDispatchToProps
doList)
rt default VisibleTodoList
This section introduces a hypothetical extension to our app that allows it to support multiple Todo Lists. Please note that a full implementation of this extension requires changes to the reducers, components, actions etc. that aren?t directly relevant to the topics discussed and have been omitted for brevity.
So far we have only seen selectors receive the Redux store state as an argument, but a selector can receive props too.
Here is an App component that renders three VisibleTodoList components, each of which has a listId prop:
components/App.jsrt React from 'react'
rt Footer from './Footer'
rt AddTodo from '../containers/AddTodo'
rt VisibleTodoList from '../containers/VisibleTodoList'
t App = () => (
iv>
<VisibleTodoList listId="1" />
<VisibleTodoList listId="2" />
<VisibleTodoList listId="3" />
div>
Each VisibleTodoList container should select a different slice of the state depending on the value of the listId prop, so let?s modify getVisibilityFilter and getTodos to accept a props argument:
selectors/todoSelectors.jsrt { createSelector } from 'reselect'
t getVisibilityFilter = (state, props) =>
ate.todoLists[props.listId].visibilityFilter
t getTodos = (state, props) =>
ate.todoLists[props.listId].todos
t getVisibleTodos = createSelector(
getVisibilityFilter, getTodos ],
isibilityFilter, todos) => {
switch (visibilityFilter) {
case 'SHOW_COMPLETED':
return todos.filter(todo => todo.completed)
case 'SHOW_ACTIVE':
return todos.filter(todo => !todo.completed)
default:
return todos
}
rt default getVisibleTodos
props can be passed to getVisibleTodos from mapStateToProps:
t mapStateToProps = (state, props) => {
turn {
todos: getVisibleTodos(state, props)
So now getVisibleTodos has access to props, and everything seems to be working fine.
But there is a problem!
Using the getVisibleTodos selector with multiple instances of the VisibleTodoList container will not correctly memoize:
containers/VisibleTodoList.jsrt { connect } from 'react-redux'
rt { toggleTodo } from '../actions'
rt TodoList from '../components/TodoList'
rt { getVisibleTodos } from '../selectors'
t mapStateToProps = (state, props) => {
turn {
// WARNING: THE FOLLOWING SELECTOR DOES NOT CORRECTLY MEMOIZE
todos: getVisibleTodos(state, props)
t mapDispatchToProps = (dispatch) => {
turn {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
t VisibleTodoList = connect(
pStateToProps,
pDispatchToProps
doList)
rt default VisibleTodoList
A selector created with createSelector has a cache size of 1 and only returns the cached value when its set of arguments is the same as its previous set of arguments. If we alternate between rendering <VisibleTodoList listId="1" /> and <VisibleTodoList listId="2" />, the shared selector will alternate between receiving {listId: 1} and {listId: 2} as its props argument. This will cause the arguments to be different on each call, so the selector will always recompute instead of returning the cached value. We?ll see how to overcome this limitation in the next section.
The examples in this section require React Redux v4.3.0 or greater
An alternative approach be found in re-reselect
To share a selector across multiple VisibleTodoList components while passing in props and retaining memoization, each instance of the component needs its own private copy of the selector.
Let?s create a function named makeGetVisibleTodos that returns a new copy of the getVisibleTodos selector each time it is called:
selectors/todoSelectors.jsrt { createSelector } from 'reselect'
t getVisibilityFilter = (state, props) =>
ate.todoLists[props.listId].visibilityFilter
t getTodos = (state, props) =>
ate.todoLists[props.listId].todos
t makeGetVisibleTodos = () => {
turn createSelector(
[ getVisibilityFilter, getTodos ],
(visibilityFilter, todos) => {
switch (visibilityFilter) {
case 'SHOW_COMPLETED':
return todos.filter(todo => todo.completed)
case 'SHOW_ACTIVE':
return todos.filter(todo => !todo.completed)
default:
return todos
}
}
rt default makeGetVisibleTodos
We also need a way to give each instance of a container access to its own private selector. The mapStateToProps argument of connect can help with this.
If the mapStateToProps argument supplied to connect returns a function instead of an object, it will be used to create an individual mapStateToProps function for each instance of the container.
In the example below makeMapStateToProps creates a new getVisibleTodos selector, and returns a mapStateToProps function that has exclusive access to the new selector:
t makeMapStateToProps = () => {
nst getVisibleTodos = makeGetVisibleTodos()
nst mapStateToProps = (state, props) => {
return {
todos: getVisibleTodos(state, props)
}
turn mapStateToProps
If we pass makeMapStateToProps to connect, each instance of the VisibleTodosList container will get its own mapStateToProps function with a private getVisibleTodos selector. Memoization will now work correctly regardless of the render order of the VisibleTodoList containers.
containers/VisibleTodoList.jsrt { connect } from 'react-redux'
rt { toggleTodo } from '../actions'
rt TodoList from '../components/TodoList'
rt { makeGetVisibleTodos } from '../selectors'
t makeMapStateToProps = () => {
nst getVisibleTodos = makeGetVisibleTodos()
nst mapStateToProps = (state, props) => {
return {
todos: getVisibleTodos(state, props)
}
turn mapStateToProps
t mapDispatchToProps = (dispatch) => {
turn {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
t VisibleTodoList = connect(
keMapStateToProps,
pDispatchToProps
doList)
rt default VisibleTodoList
Takes one or more selectors, or an array of selectors, computes their values and passes them as arguments to resultFunc.
createSelector determines if the value returned by an input-selector has changed between calls using reference equality (===). Inputs to selectors created with createSelector should be immutable.
Selectors created with createSelector have a cache size of 1. This means they always recalculate when the value of an input-selector changes, as a selector only stores the preceding value of each input-selector.
t mySelector = createSelector(
ate => state.values.value1,
ate => state.values.value2,
alue1, value2) => value1 + value2
ou can also pass an array of selectors
t totalSelector = createSelector(
state => state.values.value1,
state => state.values.value2
alue1, value2) => value1 + value2
It can be useful to access the props of a component from within a selector. When a selector is connected to a component with connect, the component props are passed as the second argument to the selector:
t abSelector = (state, props) => state.a * props.b
rops only (ignoring state argument)
t cSelector = (_, props) => props.c
tate only (props argument omitted as not required)
t dSelector = state => state.d
t totalSelector = createSelector(
Selector,
elector,
elector,
b, c, d) => ({
total: ab + c + d
defaultMemoize memoizes the function passed in the func parameter. It is the memoize function used by createSelector.
defaultMemoize has a cache size of 1. This means it always recalculates when the value of an argument changes.
defaultMemoize determines if an argument has changed by calling the equalityCheck function. As defaultMemoize is designed to be used with immutable data, the default equalityCheck function checks for changes using reference equality:
tion defaultEqualityCheck(currentVal, previousVal) {
turn currentVal === previousVal
defaultMemoize can be used with createSelectorCreator to customize the equalityCheck function.
createSelectorCreator can be used to make a customized version of createSelector.
The memoize argument is a memoization function to replace defaultMemoize.
The ...memoizeOptions rest parameters are zero or more configuration options to be passed to memoizeFunc. The selectors resultFunc is passed as the first argument to memoize and the memoizeOptions are passed as the second argument onwards:
t customSelectorCreator = createSelectorCreator(
stomMemoize, // function to be used to memoize resultFunc
tion1, // option1 will be passed as second argument to customMemoize
tion2, // option2 will be passed as third argument to customMemoize
tion3 // option3 will be passed as fourth argument to customMemoize
t customSelector = customSelectorCreator(
put1,
put2,
sultFunc // resultFunc will be passed as first argument to customMemoize
Internally customSelector calls the memoize function as follows:
omMemoize(resultFunc, option1, option2, option3)
Here are some examples of how you might use createSelectorCreator:
equalityCheck for defaultMemoizert { createSelectorCreator, defaultMemoize } from 'reselect'
rt isEqual from 'lodash.isEqual'
reate a "selector creator" that uses lodash.isEqual instead of ===
t createDeepEqualSelector = createSelectorCreator(
faultMemoize,
Equal
se the new "selector creator" to create a selector
t mySelector = createDeepEqualSelector(
ate => state.values.filter(val => val < 5),
lues => values.reduce((acc, val) => acc + val, 0)
rt { createSelectorCreator } from 'reselect'
rt memoize from 'lodash.memoize'
called = 0
t hashFn = (...args) => args.reduce(
cc, val) => acc + '-' + JSON.stringify(val),
t customSelectorCreator = createSelectorCreator(memoize, hashFn)
t selector = customSelectorCreator(
ate => state.a,
ate => state.b,
, b) => {
called++
return a + b
createStructuredSelector is a convenience function for a common pattern that arises when using Reselect. The selector passed to a connect decorator often just takes the values of its input-selectors and maps them to keys in an object:
t mySelectorA = state => state.a
t mySelectorB = state => state.b
he result function in the following selector
s simply building an object from the input selectors
t structuredSelector = createSelector(
ySelectorA,
ySelectorB,
ySelectorC,
a, b, c) => ({
a,
b,
c
)
createStructuredSelector takes an object whose properties are input-selectors and returns a structured selector. The structured selector returns an object with the same keys as the inputSelectors argument, but with the selectors replaced with their values.
t mySelectorA = state => state.a
t mySelectorB = state => state.b
t structuredSelector = createStructuredSelector({
mySelectorA,
mySelectorB
t result = structuredSelector({ a: 1, b: 2 }) // will produce { x: 1, y: 2 }
Structured selectors can be nested:
t nestedSelector = createStructuredSelector({
bA: createStructuredSelector({
selectorA,
selectorB
,
bB: createStructuredSelector({
selectorC,
selectorD
A: Check that your memoization function is compatible with your state update function (i.e. the reducer if you are using Redux). For example, a selector created with createSelector will not work with a state update function that mutates an existing object instead of creating a new one each time. createSelector uses an identity check (===) to detect that an input has changed, so mutating an existing object will not trigger the selector to recompute because mutating an object does not change its identity. Note that if you are using Redux, mutating the state object is almost certainly a mistake.
The following example defines a simple selector that determines if the first todo item in an array of todos has been completed:
t isFirstTodoCompleteSelector = createSelector(
ate => state.todos[0],
do => todo && todo.completed
The following state update function will not work with isFirstTodoCompleteSelector:
rt default function todos(state = initialState, action) {
itch (action.type) {
se COMPLETE_ALL:
const areAllMarked = state.every(todo => todo.completed)
// BAD: mutating an existing object
return state.map(todo => {
todo.completed = !areAllMarked
return todo
})
fault:
return state
The following state update function will work with isFirstTodoCompleteSelector:
rt default function todos(state = initialState, action) {
itch (action.type) {
se COMPLETE_ALL:
const areAllMarked = state.every(todo => todo.completed)
// GOOD: returning a new object each time with Object.assign
return state.map(todo => Object.assign({}, todo, {
completed: !areAllMarked
}))
fault:
return state
If you are not using Redux and have a requirement to work with mutable data, you can use createSelectorCreator to replace the default memoization function and/or use a different equality check function. See here and here for examples.
A: Check that your memoization function is compatible with your state update function (i.e. the reducer if you are using Redux). For example, a selector created with createSelector that recomputes unexpectedly may be receiving a new object on each update whether the values it contains have changed or not. createSelector uses an identity check (===) to detect that an input has changed, so returning a new object on each update means that the selector will recompute on each update.
rt { REMOVE_OLD } from '../constants/ActionTypes'
t initialState = [
text: 'Use Redux',
completed: false,
id: 0,
timestamp: Date.now()
rt default function todos(state = initialState, action) {
itch (action.type) {
se REMOVE_OLD:
return state.filter(todo => {
return todo.timestamp + 30 * 24 * 60 * 60 * 1000 > Date.now()
})
fault:
return state
The following selector is going to recompute every time REMOVE_OLD is invoked because Array.filter always returns a new object. However, in the majority of cases the REMOVE_OLD action will not change the list of todos so the recomputation is unnecessary.
rt { createSelector } from 'reselect'
t todosSelector = state => state.todos
rt const visibleTodosSelector = createSelector(
dosSelector,
odos) => {
...
You can eliminate unnecessary recomputations by returning a new object from the state update function only when a deep equality check has found that the list of todos has actually changed:
rt { REMOVE_OLD } from '../constants/ActionTypes'
rt isEqual from 'lodash.isEqual'
t initialState = [
text: 'Use Redux',
completed: false,
id: 0,
timestamp: Date.now()
rt default function todos(state = initialState, action) {
itch (action.type) {
se REMOVE_OLD:
const updatedState = state.filter(todo => {
return todo.timestamp + 30 * 24 * 60 * 60 * 1000 > Date.now()
})
return isEqual(updatedState, state) ? state : updatedState
fault:
return state
Alternatively, the default equalityCheck function in the selector can be replaced by a deep equality check:
rt { createSelectorCreator, defaultMemoize } from 'reselect'
rt isEqual from 'lodash.isEqual'
t todosSelector = state => state.todos
reate a "selector creator" that uses lodash.isEqual instead of ===
t createDeepEqualSelector = createSelectorCreator(
faultMemoize,
Equal
se the new "selector creator" to create a selector
t mySelector = createDeepEqualSelector(
dosSelector,
odos) => {
...
Always check that the cost of an alternative equalityCheck function or deep equality check in the state update function is not greater than the cost of recomputing every time. If recomputing every time does work out to be the cheaper option, it may be that for this case Reselect is not giving you any benefit over passing a plain mapStateToProps function to connect.
A: Yes. Reselect has no dependencies on any other package, so although it was designed to be used with Redux it can be used independently. It is currently being used successfully in traditional Flux apps.
If you create selectors using
createSelectormake sure its arguments are immutable. See here
A: Keep in mind that selectors can access React props, so if your arguments are (or can be made available as) React props, you can use that functionality. See here for details.
Otherwise, Reselect doesn't have built-in support for creating selectors that accepts arguments, but here are some suggestions for implementing similar functionality…
If the argument is not dynamic you can use a factory function:
t expensiveItemSelectorFactory = minValue => {
turn createSelector(
shopItemsSelector,
items => items.filter(item => item.value > minValue)
t subtotalSelector = createSelector(
pensiveItemSelectorFactory(200),
ems => items.reduce((acc, item) => acc + item.value, 0)
The general consensus here and over at nuclear-js is that if a selector needs a dynamic argument, then that argument should probably be state in the store. If you decide that you do require a selector with a dynamic argument, then a selector that returns a memoized function may be suitable:
rt { createSelector } from 'reselect'
rt memoize from 'lodash.memoize'
t expensiveSelector = createSelector(
ate => state.items,
ems => memoize(
minValue => items.filter(item => item.value > minValue)
t expensiveFilter = expensiveSelector(state)
t slightlyExpensive = expensiveFilter(100)
t veryExpensive = expensiveFilter(1000000)
A: We think it works great for a lot of use cases, but sure. See these examples.
A: For a given input, a selector should always produce the same output. For this reason they are simple to unit test.
t selector = createSelector(
ate => state.a,
ate => state.b,
, b) => ({
c: a * 2,
d: b * 3
("selector unit test", () => {
sert.deepEqual(selector({ a: 1, b: 2 }), { c: 2, d: 6 })
sert.deepEqual(selector({ a: 2, b: 3 }), { c: 4, d: 9 })
It may also be useful to check that the memoization function for a selector works correctly with the state update function (i.e. the reducer if you are using Redux). Each selector has a recomputations method that will return the number of times it has been recomputed:
e('selector', () => {
t state = { a: 1, b: 2 }
nst reducer = (state, action) => (
{
a: action(state.a),
b: action(state.b)
}
nst selector = createSelector(
state => state.a,
state => state.b,
(a, b) => ({
c: a * 2,
d: b * 3
})
nst plusOne = x => x + 1
nst id = x => x
st("selector unit test", () => {
state = reducer(state, plusOne)
assert.deepEqual(selector(state), { c: 4, d: 9 })
state = reducer(state, id)
assert.deepEqual(selector(state), { c: 4, d: 9 })
assert.equal(selector.recomputations(), 1)
state = reducer(state, plusOne)
assert.deepEqual(selector(state), { c: 6, d: 12 })
assert.equal(selector.recomputations(), 2)
Additionally, selectors keep a reference to the last result function as .resultFunc. If you have selectors composed of many other selectors this can help you test each selector without coupling all of your tests to the shape of your state.
For example if you have a set of selectors like this:
selectors.js
rt const firstSelector = createSelector( ... )
rt const secondSelector = createSelector( ... )
rt const thirdSelector = createSelector( ... )
rt const myComposedSelector = createSelector(
rstSelector,
condSelector,
irdSelector,
irst, second, third) => first * second < third
And then a set of unit tests like this:
test/selectors.js
ests for the first three selectors...
("firstSelector unit test", () => { ... })
("secondSelector unit test", () => { ... })
("thirdSelector unit test", () => { ... })
e have already tested the previous
hree selector outputs so we can just call `.resultFunc`
ith the values we want to test directly:
("myComposedSelector unit test", () => {
here instead of calling selector()
we just call selector.resultFunc()
sert(myComposedSelector.resultFunc(1, 2, 3), true)
sert(myComposedSelector.resultFunc(2, 2, 1), false)
Finally, each selector has a resetRecomputations method that sets
recomputations back to 0. The intended use is for a complex selector that may
have many independent tests and you don't want to manually manage the
computation count or create a “dummy” selector for each test.
A: Selectors created with createSelector should work just fine with Immutable.js data structures.
If your selector is recomputing and you don't think the state has changed, make sure you are aware of which Immutable.js update methods always return a new object and which update methods only return a new object when the collection actually changes.
rt Immutable from 'immutable'
myMap = Immutable.Map({
1,
2,
3
set, merge and others only return a new obj when update changes collection
newMap = myMap.set('a', 1)
rt.equal(myMap, newMap)
ap = myMap.merge({ 'a', 1 })
rt.equal(myMap, newMap)
ap, reduce, filter and others always return a new obj
ap = myMap.map(a => a * 1)
rt.notEqual(myMap, newMap)
If a selector's input is updated by an operation that always returns a new object, it may be performing unnecessary recomputations. See here for a discussion on the pros and cons of using a deep equality check like Immutable.is to eliminate unnecessary recomputations.
A: Selectors created using createSelector only have a cache size of one. This can make them unsuitable for sharing across multiple components if the arguments to the selector are different for each instance of the component. There are a couple of ways to get around this:
Create a factory function which returns a new selector for each instance of the component. There is built-in support for factory functions in React Redux v4.3 or higher. See here for an example.
Create a custom selector with a cache size greater than one.
A: Yes! They are included and referenced in package.json. They should Just Work?.
A: Try these helper functions courtesy of MattSPalmer
Enhances Reselect selectors by wrapping createSelector and returning a memoized collection of selectors indexed with the cache key returned by a custom resolver function.
Useful to reduce selectors recalculation when the same selector is repeatedly called with one/few different arguments.
Can be useful when doing very expensive computations on elements of a collection because Reselect might not give you the granularity of caching that you need. Check out the reselect-maps README for examples.
The optimizations in reselect-map only apply in a small number of cases. If you are unsure whether you need it, you don't!
MIT