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Prerequisite Reducer Concepts

As described in "Redux Fundamentals" Part 3: State, Actions, and Reducers, a Redux reducer function:

  • Should have a signature of (previousState, action) => newState, similar to the type of function you would pass to Array.prototype.reduce(reducer, ?initialValue)
  • Should be "pure", which means the reducer:
    • Does not perform side effects (such as calling API's or modifying non-local objects or variables).
    • Does not call non-pure functions (like or Math.random).
    • Does not mutate its arguments. If the reducer updates state, it should not modify the existing state object in-place. Instead, it should generate a new object containing the necessary changes. The same approach should be used for any sub-objects within state that the reducer updates.
Note on immutability, side effects, and mutation

Mutation is discouraged because it generally breaks time-travel debugging, and React Redux's connect function:

  • For time traveling, the Redux DevTools expect that replaying recorded actions would output a state value, but not change anything else. Side effects like mutation or asynchronous behavior will cause time travel to alter behavior between steps, breaking the application.
  • For React Redux, connect checks to see if the props returned from a mapStateToProps function have changed in order to determine if a component needs to update. To improve performance, connect takes some shortcuts that rely on the state being immutable, and uses shallow reference equality checks to detect changes. This means that changes made to objects and arrays by direct mutation will not be detected, and components will not re-render.

Other side effects like generating unique IDs or timestamps in a reducer also make the code unpredictable and harder to debug and test.

Because of these rules, it's important that the following core concepts are fully understood before moving on to other specific techniques for organizing Redux reducers:

Redux Reducer Basics

Key concepts:

  • Thinking in terms of state and state shape
  • Delegating update responsibility by slice of state (reducer composition)
  • Higher order reducers
  • Defining reducer initial state

Reading list:

Pure Functions and Side Effects

Key Concepts:

  • Side effects
  • Pure functions
  • How to think in terms of combining functions

Reading List:

Immutable Data Management

Key Concepts:

  • Mutability vs immutability
  • Immutably updating objects and arrays safely
  • Avoiding functions and statements that mutate state

Reading List:

Normalizing Data

Key Concepts:

  • Database structure and organization
  • Splitting relational/nested data up into separate tables
  • Storing a single definition for a given item
  • Referring to items by IDs
  • Using objects keyed by item IDs as lookup tables, and arrays of IDs to track ordering
  • Associating items in relationships

Reading List: