I was working with some code in React that had an infinite loop caused by useEffect and a function declared outside of the useEffect callback, after some research I found out that this is because on each re-render React creates a new inner function and the solution was to use the useCallback hook.
Still, I want to know why this is the behavior of objects (array, functions, objects) declared inside functions as opposed to values(strings, numbers, booleans, etc.) that seem to maintain their place in memory and aren't being created from scratch on every function call. I tried googling about it but didn't find anything.
Here's an example of what I'm referring to:
https://codesandbox.io/s/sweet-mccarthy-lf32r?file=/src/App.js
When you click the increase button, the effect that depends on the array, triggers, but the one depending on the number doesn't.
Ok, I think I got the answer, but I would like to be corrected if I'm wrong:
So for this to make sense we need to understand 3 things:
Garbage collection
Primitive vs References
How equality comparisons work
First, to address my previous statement:
I want to know why this is the behavior of objects (array, functions,
objects) declared inside functions as opposed to values(strings,
numbers, booleans, etc.) that seem to maintain their place in memory
and aren't being created from scratch on every function call.
Both primitives and references are created from scratch on every function call due to garbage collection, for more info check https://developer.mozilla.org/en-US/docs/Web/JavaScript/Memory_Management
Primitives are saved into the stack and objects are stored in the heap and the object gets "pointer" to the location in the heap. More on that in this video
The trick with the dependencies array in useEffect has to do with how equality works in JavaScript, a great article about this tell us that:
If both operands are numbers and have the same value, they are
strictly equal
If both operands are strings and have the same value,
they are strictly equal
If both operands have reference to the same object or function, they
are strictly equal
So if we have:
function App() {
const num = 1;
const arr = [1, 2, 3];
const [state, setState] = React.useState(1);
React.useEffect(() => {
console.log("updated by change of num");
}, [num]);
React.useEffect(() => {
console.log("updated by change of arr");
}, [arr]);
return (
<button onClick={() => setState((prev) => prev + 1)}>
increase {state}
</button>
);
}
The dependency array doesn't care if num has the same location in memory (which it doesn't due to being new in each function call) as the equality only checks for type and value, but it does care the place in memory for objects as for them to be equal they need to have the same reference.
Hope this is clear!
tl;dr: when you create objects / components in the render method to be passed as props / children to the rendered components, does it fail the checking of React.PureComponent and prop checks in shouldComponentUpdate?
I'm seeing patterns similar to this a lot:
render() {
const commentTrigger = <Button>Comment</Button>;
const deleteTrigger = <Button>Delete Thread</Button>;
const someComponentProps = {
prop1: this.getProp1()
prop2: this.props.prop2
}
return (
<div>
<SomeModal trigger={commentTrigger} />
<SomeOtherModal trigger={deleteTrigger} />
<SomeComponent {...someComponentProps}
</div>
)
}
would this fail the .PureComponent check or shouldComponentUpdate check in SomeComponent, since someComponentProps is a different object every time, and in particular (assuming this.getProp1() is a newly created object) would prop1 alone cause these tests to fail and end up re-rendering the component? How about if this.getProp1() is just a number?
Otherwise, is there any good reason to not write React components in this way?
Thanks for your help in advance. Let me know if I should rephrase the question or clarify anything.
PureComponent will do a shallow comparison of its props, basically using the Object.is() comparison. In the case of objects, that means it will only pass if it's the same object, not if it's a different object with all the same properties.
So your first two examples (commentTrigger and deleteTrigger) are going to run into this. They're brand new objects each time you render, unrelated to the previous ones, and thus will not pass a triple equals.
In your third example (spreading someComponentProps), it makes no difference whether someComponentProps is a new object or not. By spreading it, you are passing in a sequence of individual props, starting with prop1={someComponentProps.prop1} and running through the rest of the properties on the object. It's a convenient syntax when you don't know how many props there are. If SomeComponent is a pure component, it will check whether prop1 changed and whether prop2 changed, with no idea that someComponentProps even existed.
PureComponent does shallow equality check of props object. In case next prop1 value (created with this.getProp1()) is === equal to previous value, so is prop2, SomeComponent won't be updated.
Even if props aren't === equal, custom checks can be implemented in shouldComponentUpdate, e.g. deep equality check.
Started learning react but made my state as an array.
my state was not getting properly update when i did something like
this.setState(state => [newItem, ...this.state])
above statement was converting an array to integer indexed object
I was getting a warning
index.js:2178 Warning: App.state: must be set to an object or null
seems like there is some check in react-dom code like this
if (state && (typeof state !== 'object' || isArray(state))) {
link
why is this happening
what are the issues i can face if i use the state as an array or something else than object or null
setState accepts objects, plain and simple. If you call this.setState(['a','b']) it will convert your array to an array-like object. Why? Because React allows that elsewhere in your code you should be able to call this.setState({data: 'something'}) without having that fail. For reference this.state would now look like this:
{
0: 'a',
1: 'b',
data: 'something'
}
Why did you get a warning? Because React performed this conversion, from array to array-like object, behind the scenes, and wants to let you know that things have changed. For example, after setting state to an array, you won't be able to call this.state.map ... or any other array methods.
If you need to store an array in state, set it to an object property: this.setState({arrayData: ['a', 'b']}). In general it's a good practice to wrap your data in object properties, because you will certainly be lifting state up as you develop your application.
We have a project using React + Redux + ImmutableJS. One of our engineers recently added a helper method to support destructuring an ImmutableJS Map when passing it to a component as props:
export function toObjectShallow(mapping: Map) {
const result = {};
mapping.map((value, key) => {
result[key] = value;
});
return result;
}
Thus, we can still do the following and avoid being verbose with repeated calls to Map.get:
<XyzComponent {...toObjectShallow(this.props.xyz)}/>
Yes, that's essentially making two shallow copies (our method + destructuring) of the original object. That should be of minimal expense. I'm wondering though, since I don't see this kind of recommendation really anywhere else in the React/Redux/Immutable communities, is there something else I'm missing that would make this unideal?
The passed properties are still the original, immutable props. It's just the containing object is mutated which doesn't matter, because it's not getting passed to the component anyways. So, what gives? This seems like such a simple solution while avoiding toJS(). Why isn't it really mentioned anywhere?
I followed the advice in the redux docs to use a HOC for all connected components to allow me to interact with plain javascript objects outside of redux. So my selectors and reducers still use ImmutableJS objects, but the rest of the code uses plain javascript objects:
https://redux.js.org/docs/recipes/UsingImmutableJS.html#use-a-higher-order-component-to-convert-your-smart-components-immutablejs-props-to-your-dumb-components-javascript-props
edit- not sure if this is the toJS you are mentioning above, I had assumed you meant ImmutableJS.toJS.
as far as preferences, using an HOC you only need to do it once per component, as opposed to each time you use a component in your method.
I think the "correct" answer I was looking for was to continue using Immutable JS's toObject() which does a shallow conversion of an Immutable Map object to a regular object, thus allowing us to continue using syntactic sugar while keeping the properties immutable and not having to use our own shallow copy.
Whilst learning Redux I've came across Reducers. The documentation states:
The reducer is a pure function that takes the previous state and an action, and returns the next state. (previousState, action) => newState. It's called a reducer because it's the type of function you would pass to Array.prototype.reduce(reducer, ?initialValue) .
MDN describes the reduce method as:
The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.
I'm still confused on why the Redux definition of a reducer as it's making no sense. Secondly the MDN description doesn't seem correct either. The reduce method isn't always used to reduce to a single value. It can be used in place of map and filter and is actually faster when used in place of chaining.
Is the MDN description incorrect?
Jumping back to the Redux definition of a reducer, it states:
It's called a reducer because it's the type of function you would pass to Array.prototype.reduce(reducer, ?initialValue)
I'm under the impression that a reducer in Redux is responsible for modifying state. An example reducer:
const count = function(state, action) {
if(action.type == 'INCREMENT') {
return state + 1;
} else if(action.type == 'DECREMENT') {
return state - 1;
} else {
return state;
}
}
... I don't see how this is a function that would be passed to reduce. How is that data being reduced to a single value? If this is a function you would pass to reduce then state would be the callback and action would be the initial value.
Thanks for any clear explanations. It's difficult to conceptualize.
The term "reduce" is actually a functional term used in functional programming. In a language like Haskell, F# or even JavaScript, we define a transformation that takes a collection (of any size) as input and returns a single value as output.
So (not to be pedantic, but I find this helps me) think of it visually. We have a collection:
[][][][][][][][][][]
...which we want to collapse into a single value:
N
Programming functionally, we would do this with a single function that we could call recursively on each element of the collection. But if you do that, you need to keep track of the intermediate value somewhere, right? Non-pure implementations might keep some kind of "accumulator" or variable outside of the function to keep track of the state, like so:
var accumulator = 0;
var myArray = [1,2,3,4,5];
myArray.reduce(function (each) {
accumulator += 0;
});
return accumulator;
With pure functions, though, we can't do this - because by definition, pure functions can't have effects outside of their function scope. Instead of relying on an external variable that encapsulates our "state" between calls, we simply pass the state along in the method:
var myArray = [1,2,3,4,5];
return myArray.reduce(function (accumulator, each) {
return accumulator + each;
}, 0);
In this case we call the function a "reducer" because of its method signature. We have each (or current - any name is fine), representing an object in the collection; and state (or previous), which is passed to each iteration of the function, representing the results of the transformation we've already done to the previous elements in the collection.
Note that the MDN documentation you referenced is correct; the reduce() function always does return a single value. In fact, the reduce method in any language is a higher-order function that takes a "reducer" (a function with the method signature defined above) and returns a single value. Now, yes, you can do other stuff with it, if your function that you call has side effects, but you shouldn't. (Essentially, don't use .reduce() as a foreach.) Even if the method you call with reduce has side effects, the return value of reduce itself will be a single value, not a collection.
The cool thing is, this pattern doesn't just have to apply to arrays or concrete collections, as you've seen in React; this pattern can be applied to streams as well, since they're pure functions.
Hope this helps. For what it's worth, the definition on the Redux site could be improved (as the concept of a reducer isn't just because of Javascript's Array prototype method). You should submit a PR!
Edit: There's a Wikipedia article on the subject. Note that reduce has different names, and in functional languages, it's commonly known as Fold. https://en.wikipedia.org/wiki/Fold_(higher-order_function)#Folds_as_structural_transformations
Edit (2020-10-03): People still seem to be finding this useful - that's good. With time, I've realized that "fold" is a much better term for this; the functional languages got it right. "Reducer" isn't really a bad term, but it's not necessarily a good one, either.
The reason why a redux reducer is called a reducer is because you could "reduce" a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.
How? To answer that, let me define a reducer again:
The reduce() method applies a function (reducer) against an accumulator and each
value of the array (from left-to-right) to reduce it to a single
value.
And what does a redux reducer do?
The reducer is a pure function that takes the current state and an
action, and returns the next state. Note that the state is accumulated as each action on the collection is applied to change this state.
So given a collection of actions, the reducer is applied on each value of the collection (from left-to-right). The first time, it returns the initial value. Now the reducer is applied again on this initial state and the first action to return the next state. And the next collection item (action) is applied each time on the current state to get the next state until it reaches the end of the array. And then, you get the final state. How cool is that!
Sorry, but I would disagree with previous answers. I would not support the naming reducer. I'm passionate about FP and immutability. Don't blame me, read the second part, but I want to state first, why I disagree.
It's correct that the reducers are the sequence of transformations, but the sequence itself - could be part of another sequence. Imagine it, like links - a part of chain. But the chain itself could be part of longer chain. Each link is the "transition" of the global state. Than, what the theory behind it?
Isn't it actually the "Finite state machine"? - close, but not. It's actually the Transition system.
A labelled transition system is a tuple (S, Λ, →) where S is a set of states, Λ is a set of labels and → is a set of labelled transitions
So, S - are set of our states
Λ - is our so-called "actions" (but labels in theory)
... and
→ - reducers "labelled transitions"! I would name it so, if I am creator of this library.
Understanding this theory helped me to implement my library, where I can have low-level transition system as a part of high-level transition system (like chain - still could be part of longer chain) - and still having single global Redux state.
Calling Redux reducers reducers is semantically incorrect and doesn't make much sense.
That's why the author is confused.
A reducer is a function that reduces a set of values to a single value.
We can also say that it folds the values - thus the classic fold() fn in functional programming.
Since Redux reducer does not fold a set of value, but applies an action to a state and always returns the same shape (State -> Action -> State) - it should be called applicator or applier.
But, since we have to always return the same shape of the state, and not just smth completely unrelated - we'd make much more sense calling Redux state applicators - changers, transformers or mutators.
And, it has indeed become commonplace to use terms like 'mutate the state' and 'state mutator'.
But Redux sounds just so much cooler, than Applux or Mutux :)
I'm under the impression that a reducer in Redux is responsible for modifying state. An example reducer:
const count = function(state, action) {
if (action.type == 'INCREMENT') {
return state + 1;
} else if (action.type == 'DECREMENT') {
return state - 1;
} else {
return state;
}
}
... I don't see how this is a function that would be passed to reduce. How is that data being reduced to a single value? If this is a function you would pass to reduce then state would be the callback and action would be the initial value.
// count function from your question
const count = function (state, action) {
if (action.type == 'INCREMENT') {
return state + 1;
} else if (action.type == 'DECREMENT') {
return state - 1;
} else {
return state;
}
}
// an array of actions
const actions =
[ { type: 'INCREMENT' }
, { type: 'INCREMENT' }
, { type: 'INCREMENT' }
, { type: 'INCREMENT' }
, { type: 'DECREMENT' }
]
// initial state
const init = 0
console.log(actions.reduce(count, init))
// 3 (final state)
// (INCREMENT 4 times, DECREMENT 1 time)
These answers are good but I think it's much simpler than you think. I'll admit that I had a similar confusion at first. The reduce method on Javascript arrays takes an accumulator and a callback function.
const arr = [1, 2, 3]
const sum = arr.reduce((accumulator, element) => {
accumulator += element;
return accumulator;
}); // sum equals 6 now
The reason it's called a reducer in redux is because it roughly has a similar structure.
const sum = arr.reduce((accumulator, element) => { // accumulator is the initial state
accumulator += element; // we do something to modify the initial state
return accumulator; // we return that and it becomes the new state
});
So every time we run a reducer we take something in, modify it, and return a copy of that same thing. On each iteration we're pointing to the same thing. Ok, yes, we have to make a copy in redux so as to not directly modify state, but symbolically every time we run it, it's kind of like the way reduce starts with an initial state in the example above of 1. Then we add 2 to our inital state and return 3. Now we run our 'reducer' again with an intial state of 3 and add 3 and we end up with 6.
It's called a reducer because it's the type of function you would pass to Array.prototype.reduce(reducer, ?initialValue)
Array.reduce
This is very similiar to what you would pass to Array.reduce as the callback (reducer). The important part being:
callback
Function to execute on each value in the array, taking four arguments:
previousValue
The value previously returned in the last invocation of the callback, or initialValue, if supplied. (See below.)
currentValue
The current element being processed in the array.
Where state is the "previousValue" and action is the "currentValue".
Both Array.reduce and Redux take a "reducer" function, which reduces previous and current into a single return value.
const reducer = (previous, current) => {
/* calculate value */
return value;
};
previous
current
return
Array.reduce
accumulator
current element
new accumulator
Redux
state
action
new state
Redux docs:
If we were to create an array of Redux actions, call reduce(), and
pass in a reducer function, we'd get a final result the same way:
const actions = [
{ type: 'counter/incremented' },
{ type: 'counter/incremented' },
{ type: 'counter/incremented' }
]
const initialState = { value: 0 }
const finalResult = actions.reduce(counterReducer, initialState)
console.log(finalResult)
// {value: 3}
We can say that Redux reducers reduce a set of actions (over time)
into a single state. The difference is that with Array.reduce() it
happens all at once, and with Redux, it happens over the lifetime of
your running app.
It's a confusing name because in practice it has nothing to do with Array.prototype.reduce.
In the Redux source code here, this is how your reducer is called:
currentState = currentReducer(currentState, action)
It's enticing to use the term because your state at any point in time could be found by calling arrayOfPreviousActions.reduce(reducer).
['INCREMENT', 'INCREMENT'].reduce((currentState, action) => currentState++, 0)
But it's not actually called like this.
The real question is what would be a better name for this?
If we don't consider the output/purpose, it's handling an action/event. An actionHandler, actionProcessor.
Aside: event is probably a better term than action too because Redux is often also used to respond to API responses which results in action types set to GET_USERS_SUCCESS...which is clearly an event. This was simply adopted from Flux though.
Redux draws inspiration from the Event Sourcing pattern and State Machines/Transition Systems. Googling around revealed the term state transition often used.
state transitions can be represented as functions that take a state and an event, and produce a new state
But what do we call these "functions"?
IMHO, reducer is a poor name because it doesn't encode any meaning about its purpose, just what it looks like, but I can't think of a better one.
In the end, all names for things were invented at some point, and given its popularity, reducer is now associated with state reducer or state transition function.
Just look at the origin of the term argument for example:
The use of the term "argument" developed from astronomy, which historically used tables to determine the spatial positions of planets from their positions in the sky. These tables were organized according to measured angles called arguments, literally "that which elucidates something else."
It's not a very specific name, but the reducer in Redux does the same thing that other reducer functions do. They take in multiple things and give us back one thing. The Redux reducer takes in two things (the previous state and an action to be performed on that previous state) and reduce those two things down to one: the next state.