Effects for side effects in Angular - Step-by-Step Execution

Concept Flow - Effects for side effects

Action Dispatched

↓

Effect Listens

↓

Perform Side Effect

↓

Dispatch New Action

↓

Store Updates State

↓

UI Reacts to State Change

When an action happens, an effect listens and runs a side effect like fetching data, then sends a new action to update the state and UI.

Execution Sample

Angular

readonly loadData$ = createEffect(() =>
  this.actions$.pipe(
    ofType(loadData),
    switchMap(() => this.api.getData()),
    map(data => loadDataSuccess({ data }))
  )
);

This effect listens for 'loadData' action, fetches data from API, then dispatches 'loadDataSuccess' with the data.

Execution Table

Step	Action Dispatched	Effect Triggered	Side Effect	New Action Dispatched	Store Update
1	loadData	Yes	API call started	No	No
2	API call resolves	No	Data received	loadDataSuccess({ data })	No
3	loadDataSuccess({ data })	No	No side effect	No	State updated with data
4	UI updates	No	No	No	UI shows new data
5	No more actions	No	No	No	Idle
Exit	No new actions	No	No	No	End of effect cycle

💡 No new actions dispatched, effect cycle ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
actions$	idle	loadData dispatched	API response received	loadDataSuccess dispatched	idle
state.data	empty	empty	empty	filled with API data	filled with API data
UI	empty view	loading indicator	loading indicator	data displayed	data displayed

Key Moments - 3 Insights

Why does the effect dispatch a new action instead of directly updating the state?

Effects only handle side effects and produce new actions; the store updates state when it receives these actions, as shown in steps 2 and 3 of the execution_table.

What happens if the API call fails inside the effect?

You would handle errors by dispatching a failure action inside the effect, similar to how loadDataSuccess is dispatched, but with error info. This is not shown here but follows the same pattern.

Does the effect run on every action?

No, it only runs when the specific action it listens for is dispatched, like loadData in step 1.

Visual Quiz - 3 Questions

Look at the execution_table, what action triggers the effect to start the API call?

A loadDataSuccess

B loadData

C API response received

D UI updates

💡 Hint

Check Step 1 in the execution_table where the effect triggers on an action.

At which step does the store update its state with the new data?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look for 'State updated with data' in the Store Update column.

If the effect did not dispatch a new action after the API call, what would happen?

A The store would update automatically

B The UI would show loading forever

C The state would not update with new data

D The effect would run twice

💡 Hint

Refer to how the store updates only after receiving a new action in the execution_table.

Concept Snapshot

Effects listen for specific actions.
They run side effects like API calls.
After side effects, they dispatch new actions.
Store updates state on these new actions.
UI reacts to state changes.
This keeps side effects separate from state logic.

Full Transcript

In Angular, effects listen for actions dispatched to the store. When an action like 'loadData' is dispatched, the effect triggers and performs a side effect such as calling an API. Once the API responds, the effect dispatches a new action, for example 'loadDataSuccess', carrying the data. The store listens for this new action and updates its state accordingly. The UI then updates to reflect the new state. This flow separates side effects from state management, keeping code clean and predictable.

Practice

1. What is the main purpose of Effects in Angular applications?

easy

A. To define routes in the application

B. To style components dynamically

C. To handle side tasks like data fetching or logging outside components

D. To manage component templates

Solution

1. Step 1: Understand the role of Effects

   Effects are designed to handle side effects such as data fetching or logging, which are tasks outside the component's direct responsibilities.

2. Step 2: Compare with other options

   Styling, template management, and routing are handled by other Angular features, not Effects.

3. Final Answer:

   To handle side tasks like data fetching or logging outside components -> Option C

4. Quick Check:

   Effects = side tasks handler [OK]

Hint: Effects manage side tasks outside components [OK]

Common Mistakes:

• Confusing Effects with component styling
• Thinking Effects manage routing
• Assuming Effects handle templates

2. Which of the following is the correct way to create an effect using Angular's createEffect function?

easy

A. const loadData$ = createEffect(() => { this.actions$.pipe(ofType(load), switchMap(() => fetchData())); });

B. const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), switchMap(() => fetchData())));

C. const loadData$ = createEffect(this.actions$.pipe(ofType(load), switchMap(() => fetchData())));

D. const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), map(() => fetchData())));

Solution

1. Step 1: Check the syntax of createEffect

   The createEffect function expects a function returning an observable, so it should be () => this.actions$.pipe(...).

2. Step 2: Verify operators used

   Using switchMap is correct for side effects that return new observables. const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), switchMap(() => fetchData()))); uses this correctly.

3. Final Answer:

   const loadData$ = createEffect(() => this.actions$.pipe(ofType(load), switchMap(() => fetchData()))); -> Option B

4. Quick Check:

   createEffect needs a function returning observable [OK]

Hint: createEffect needs a function returning an observable [OK]

Common Mistakes:

• Passing observable directly instead of a function
• Using map instead of switchMap for async calls
• Not returning the observable inside createEffect

3. Given this effect code snippet:

loadData$ = createEffect(() => this.actions$.pipe(
  ofType(loadData),
  switchMap(() => this.api.getData()),
  map(data => loadDataSuccess({ data })),
  catchError(() => of(loadDataFailure()))
));

What happens when the loadData action is dispatched?

medium

A. The API call is ignored and no action is dispatched

B. The effect causes a syntax error and stops

C. Only loadDataFailure action is dispatched immediately

D. The API call is made, and on success, loadDataSuccess action is dispatched

Solution

1. Step 1: Understand the effect flow

   When loadData action is dispatched, the effect listens and triggers the API call via switchMap.

2. Step 2: Analyze success and error handling

   On success, map dispatches loadDataSuccess with data; on error, catchError dispatches loadDataFailure.

3. Final Answer:

   The API call is made, and on success, loadDataSuccess action is dispatched -> Option D

4. Quick Check:

   Effect triggers API and dispatches success or failure [OK]

Hint: Effects dispatch success or failure actions after API calls [OK]

Common Mistakes:

• Assuming no action is dispatched after API call
• Confusing map with catchError behavior
• Thinking effect causes syntax error

4. Identify the error in this effect code:

saveData$ = createEffect(() => this.actions$.pipe(
  ofType(saveData),
  switchMap(action => this.api.save(action.payload)),
  map(() => saveDataSuccess()),
  catchError(error => saveDataFailure({ error }))
));

medium

A. The catchError operator should return an observable

B. The switchMap should not use action parameter

C. The map operator must return the original action

D. The effect should not use createEffect function

Solution

1. Step 1: Check catchError usage

   The catchError operator must return an observable, but here it returns an action object directly.

2. Step 2: Correct catchError return

   Wrapping the action in of() makes it an observable, fixing the error.

3. Final Answer:

   The catchError operator should return an observable -> Option A

4. Quick Check:

   catchError must return observable [OK]

Hint: Always wrap catchError return in of() to return observable [OK]

Common Mistakes:

• Returning plain object instead of observable in catchError
• Misusing switchMap parameters
• Thinking map must return original action

5. You want to create an effect that listens for login actions, calls an API to authenticate, and then dispatches either loginSuccess or loginFailure. Additionally, you want to log every login attempt regardless of success or failure. Which approach correctly implements this using Angular Effects?

hard

A. Use two separate effects: one for API call with dispatch, another for logging with dispatch: false

B. Use one effect with switchMap for API call and tap for logging inside the same pipe

C. Use one effect with map for API call and catchError for logging

D. Use one effect with filter to block logging and API call

Solution

1. Step 1: Separate concerns for side effects

   Logging is a side effect that does not dispatch actions, so it should be in a separate effect with dispatch: false.

2. Step 2: API call effect dispatches success or failure

   The main effect handles the API call and dispatches loginSuccess or loginFailure accordingly.

3. Step 3: Final design

   Two effects keep code clean and responsibilities clear: one for API calls with dispatch, one for logging without dispatch.

4. Final Answer:

   Use two separate effects: one for API call with dispatch, another for logging with dispatch: false -> Option A

5. Quick Check:

   Separate effects for dispatching and logging [OK]

Hint: Use separate effects for dispatching and non-dispatching tasks [OK]

Common Mistakes:

• Combining logging and dispatching in one effect incorrectly
• Using map instead of switchMap for API calls
• Forgetting dispatch: false for logging effect