Ronaldo Vitto Lewerissa

Software engineering learning documentation.

Why Redux Use Pure Function

Before going straight to the question, let us steal a piece of code from Redux that features combineReducers() function.

It seems that my previous understanding on how Redux implements combineReducers() does not aligns with the current implementation.

Here's my prior understanding on how it works:

const combineReducers = (reducers) => {  
  return (state = {}, action) => {
    return Object.keys(reducers).reduce(
     (nextState, key) => {
       nextState[key] = reducers[key](
       return nextState;

Current Implementation:

var reducerKeys = Object.keys(reducers);  
var finalReducers = {};  
for (var i = 0; i < reducerKeys.length; i++) {  
  var key = reducerKeys[i];
  finalReducers[key] = reducers[key];
var finalReducerKeys = Object.keys(finalReducers);

return function combination() {  
    var state = arguments[0];
    var action = arguments[1];

    var hasChanged = false;
    var nextState = {};
    for (var i = 0; i < finalReducerKeys.length; i++) {
      var key = finalReducerKeys[i];
      var reducer = finalReducers[key];
      var previousStateForKey = state[key];
      var nextStateForKey = reducer(previousStateForKey, action);
      if (typeof nextStateForKey === 'undefined') {
        var errorMessage = getUndefinedStateErrorMessage(key, action);
        throw new Error(errorMessage);
      nextState[key] = nextStateForKey;
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey;
    return hasChanged ? nextState : state;

Note: several lines of code are stripped for simplicity sake.

As you can see it is no longer using the reduce() as it was.

It is making certain optimization by considering whether the new state of the corresponding reducer has changed.

If one of the state changes, then it will return the whole new state. Otherwise if none changes ( nextStateForKey !== previousStateForKey keeps returning false ), it will return the previous state, and I assume there won't be any re-rendering.

Redux does this to ensure efficiency. Why re-renders if the state is equivalent as it was.

Due it's deterministic behavior, it will outputs the same result with the same input.

This leads to the question...

Why Pure Function?

As you saw in the preceding code, the combination() determines whether returning a new state is necessary through === operator, which is far efficient than a deep comparison.

Making any mutation to a state (and returning the mutated state) will make the comparison (hasChanged) results to false, which is not something we want to have (because a change HAS occur!).


One of the characteristic of pure function is it's prohibition to
mutate the arguments the function receives.

It matches exactly on how a reducer needs to be designed!

Every time a changes is made upon a state, a reducer has to return a new fresh state, and NOT making changes to the argument (previous state), therefore making it easier for Redux to tell whether it a change has occur.

It's that simple.

Written by Ronaldo Vitto Lewerissa

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