Why is a Redux reducer called a reducer?

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.

> 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.

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 :)

> 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".

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."

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 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.