Actions and reducers pattern in Angular - Performance & Optimization

Performance: Actions and reducers pattern

MEDIUM IMPACT

This pattern affects how state changes trigger UI updates and how efficiently Angular processes those changes during user interactions.

Updating application state on user actions

Angular

const reducer = (state, action) => {
  switch(action.type) {
    case 'UPDATE':
      return { ...state, data: action.payload };
    default:
      return state;
  }
};

Using shallow copies and simple state updates reduces computation and Angular's change detection workload.

📈 Performance GainReduces UI blocking time by 50% or more, improving input responsiveness.

Updating application state on user actions

Angular

const reducer = (state, action) => {
  switch(action.type) {
    case 'UPDATE':
      // heavy computation or deep cloning
      const newState = JSON.parse(JSON.stringify(state));
      newState.data = action.payload;
      return newState;
    default:
      return state;
  }
};

Deep cloning state on every action causes slow reducer execution and triggers excessive Angular change detection cycles.

📉 Performance CostBlocks UI thread for tens of milliseconds on each update, increasing INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy deep cloning in reducers	Triggers full component tree checks	Multiple reflows if many bindings update	High paint cost due to many DOM updates	[X] Bad
Shallow state updates with OnPush	Minimal DOM updates, only changed components	Single reflow per update	Low paint cost	[OK] Good

Rendering Pipeline

Actions dispatched update the state via reducers, triggering Angular's change detection which recalculates templates and updates the DOM.

→Change Detection

→Template Rendering

→DOM Update

⚠️ BottleneckChange Detection when reducers cause large or unnecessary state changes

Core Web Vital Affected

INP

This pattern affects how state changes trigger UI updates and how efficiently Angular processes those changes during user interactions.

Optimization Tips

1Avoid heavy computations or deep cloning inside reducers.

2Use shallow copies to update only changed state parts.

3Combine reducers with OnPush change detection for best UI responsiveness.

Performance Quiz - 3 Questions

What is a main performance risk when reducers perform deep cloning of state objects?

A It reduces bundle size

B It blocks the UI thread causing slow input responsiveness

C It improves rendering speed

D It prevents change detection

DevTools: Performance

How to check: Record a performance profile while interacting with the app, then analyze the Change Detection and scripting time.

What to look for: Look for long scripting tasks during state updates and excessive change detection cycles indicating inefficient reducers.

Practice

1. In Angular's actions and reducers pattern, what is the main role of an action?

easy

A. To describe what happened and carry data about the event

B. To directly update the UI components

C. To store the entire application state

D. To fetch data from the server

Solution

1. Step 1: Understand the purpose of actions

   Actions are simple objects that describe an event that happened in the app and carry any necessary data.

2. Step 2: Differentiate from other parts

   Reducers handle state changes, not actions. UI updates and data fetching are separate concerns.

3. Final Answer:

   To describe what happened and carry data about the event -> Option A

4. Quick Check:

   Action = event description + data [OK]

Hint: Actions describe events, reducers change state [OK]

Common Mistakes:

• Confusing actions with reducers
• Thinking actions update UI directly
• Assuming actions hold the whole state

2. Which of the following is the correct syntax to define an action using Angular's createAction function?

easy

A. const loadItems = createAction('Load Items', payload);

B. const loadItems = createAction = 'Load Items';

C. const loadItems = actionCreate('Load Items');

D. const loadItems = createAction('Load Items');

Solution

1. Step 1: Recall createAction syntax

   The correct syntax is calling createAction with a string describing the action type.

2. Step 2: Check each option

   const loadItems = createAction('Load Items'); matches the correct syntax. const loadItems = createAction = 'Load Items'; uses wrong assignment. const loadItems = actionCreate('Load Items'); uses wrong function name. const loadItems = createAction('Load Items', payload); incorrectly adds a second argument without proper structure.

3. Final Answer:

   const loadItems = createAction('Load Items'); -> Option D

4. Quick Check:

   createAction('type') is correct [OK]

Hint: createAction takes a single string type [OK]

Common Mistakes:

• Using wrong function names
• Assigning createAction instead of calling it
• Passing payload directly as second argument

3. Given this reducer snippet, what will be the new state after dispatching { type: 'increment' } if the initial state is { count: 0 }?

function counterReducer(state = { count: 0 }, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    default:
      return state;
  }
}

medium

A. { count: 1 }

B. { count: 0 }

C. { count: -1 }

D. undefined

Solution

1. Step 1: Identify the action type and initial state

   The action type is 'increment' and initial state has count 0.

2. Step 2: Follow reducer logic for 'increment'

   The reducer returns a new state with count increased by 1, so count becomes 1.

3. Final Answer:

   { count: 1 } -> Option A

4. Quick Check:

   increment adds 1 to count [OK]

Hint: Reducer returns new state based on action type [OK]

Common Mistakes:

• Returning old state instead of updated
• Confusing increment with decrement
• Expecting mutation instead of new object

4. What is wrong with this reducer code snippet?

function todoReducer(state = [], action) {
  if (action.type = 'add') {
    return [...state, action.payload];
  }
  return state;
}

medium

A. State should be an object, not an array

B. Missing default case in the reducer

C. Using assignment (=) instead of comparison (===) in the if condition

D. Reducer should not return a new array

Solution

1. Step 1: Check the if condition syntax

   The condition uses single equals (=) which assigns instead of compares. This causes a bug.

2. Step 2: Verify other parts

   Default case is handled by returning state. State as array is valid for todo list. Returning new array is correct for immutability.

3. Final Answer:

   Using assignment (=) instead of comparison (===) in the if condition -> Option C

4. Quick Check:

   Use '===' for comparison in conditions [OK]

Hint: Use '===' for comparisons, not '=' [OK]

Common Mistakes:

• Confusing '=' with '===' in conditions
• Thinking default case is missing
• Believing state must be an object

5. You want to add a new feature to reset the counter state to zero using actions and reducers. Which of these is the best way to implement the reset action and update the reducer accordingly?

hard

A. Add case 'reset': state.count = 0; return state; directly in reducer without action

B. Define const reset = createAction('reset'); and add case 'reset': return { count: 0 }; in reducer

C. Define const reset = createAction('reset', () => 0); and return 0 in reducer

D. Use dispatch({ type: 'reset', count: 0 }) and ignore reducer changes

Solution

1. Step 1: Define the reset action properly

   Use createAction with a string type 'reset' to define the action.

2. Step 2: Update reducer to handle reset

   Add a case for 'reset' that returns a new state object with count set to 0, ensuring immutability.

3. Final Answer:

   Define const reset = createAction('reset'); and add case 'reset': return { count: 0 }; in reducer -> Option B

4. Quick Check:

   Action + reducer case resets state immutably [OK]

Hint: Create action and return new state in reducer [OK]

Common Mistakes:

• Mutating state directly in reducer
• Ignoring reducer update for new action
• Misusing createAction with payload function