Does Redux Compare States By Reference or Deep?

Understanding how Redux compares state changes is crucial for optimizing performance and ensuring your React applications behave as expected. This article delves into the mechanics of Redux state comparison, clarifying whether it utilizes reference or deep equality checks.

Redux State Comparison: A Shallow Approach

Redux employs a shallow comparison to determine if the state has changed. This means it checks if the previous state object and the new state object refer to the same memory location. It doesn’t delve into the nested properties of the objects.

Let’s illustrate this with a JavaScript example:

// By Reference
let first = { a: 1 };
let second = first; // second points to the same memory location as first

first.a = 2; 
console.log(first === second); // true (both variables reference the same object)


// By Value (using the spread operator)
let firstValue = { a: 1 };
let secondValue = { ...firstValue }; // creates a new object with the same values

firstValue.a = 2;
console.log(firstValue === secondValue); // false (different objects in memory) 
console.log(secondValue.a) //1 (original object remains unchanged)

As demonstrated, modifying first also changes second because they share the same reference. However, using the spread operator (...) in the second example creates a new object, resulting in firstValue and secondValue being distinct.

Redux reducers leverage this principle. They should always return a new state object when a change occurs, ensuring a different memory reference.

const initialState = {
  data: [],
  isError: false,
  isLoading: true
};

function exampleReducer(state = initialState, action) {
  switch (action.type) {
    case 'FETCH_DATA':
      return { ...state, isLoading: true }; // New state object
    case 'DATA_RECEIVED':
      return { ...state, isLoading: false, data: action.payload }; // New state object
    default:
      return state; 
  }
}

Redux then performs a simple reference equality check:

oldState === newState; // true or false

If false (meaning a new object was returned by the reducer), Redux knows the state has changed and triggers a re-render of the necessary components. This shallow comparison is significantly faster than a deep comparison, enhancing performance.

Handling Deeply Nested Objects

Shallow comparison works efficiently for most cases. However, challenges arise with deeply nested objects. Updating nested properties without creating entirely new objects at each level can become complex.

Two common solutions address this:

1. State Normalization: Restructure your state into a flatter format, often using unique IDs as keys. This simplifies updates and improves efficiency.

2. Immutable Data Structures: Utilize libraries like Immutable.js, which provide efficient ways to update nested data without mutation. These libraries ensure that any modification creates a new object, maintaining referential equality for unchanged parts of the state.

Why Shallow Comparison?

Redux’s choice of shallow comparison boils down to performance. Deep comparison, especially with complex state trees, can be computationally expensive. Shallow comparison provides a quick and efficient way to detect changes, making Redux suitable for managing large and dynamic applications. Additionally, using pure functions (reducers that don’t mutate the state) enhances predictability and testability.

Conclusion

Redux compares states by reference, utilizing a shallow equality check for optimal performance. While this approach works well for most scenarios, consider state normalization or immutable data structures for managing deeply nested objects effectively. Understanding this fundamental aspect of Redux is vital for building performant and maintainable React applications.