Computed signals for derived values in Angular - Step-by-Step Execution

Concept Flow - Computed signals for derived values

Create base signals

↓

Define computed signal

↓

Access computed signal

↓

Base signal changes?

No→Use cached value

Yes↓

Recompute derived value

↓

Update computed signal value

This flow shows how base signals are created, a computed signal is defined from them, and how changes in base signals trigger recomputation of the derived value.

Execution Sample

Angular

const count = signal(1);
const doubleCount = computed(() => count() * 2);
console.log(doubleCount());
count.set(3);
console.log(doubleCount());

This code creates a base signal 'count', a computed signal 'doubleCount' that doubles 'count', then logs the computed value before and after changing 'count'.

Execution Table

Step	Action	count value	doubleCount value	Explanation
1	Initialize count with 1	1	undefined	Base signal 'count' starts at 1, computed not evaluated yet
2	Define doubleCount as count * 2	1	undefined	Computed signal created but not yet accessed
3	Access doubleCount()	1	2	Computed runs: 1 * 2 = 2, caches value 2
4	Set count to 3	3	2	Base signal changes, computed cache invalidated
5	Access doubleCount() again	3	6	Computed re-runs: 3 * 2 = 6, updates cache

💡 Execution stops after logging updated computed value; no further changes.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5
count	1	1	3	3
doubleCount	undefined	2	2 (cache invalidated)	6

Key Moments - 2 Insights

Why does doubleCount not update immediately when count changes?

The computed signal caches its value until accessed again. When 'count' changes (Step 4), the cache is invalidated but 'doubleCount' only recomputes when called next (Step 5). See execution_table rows 4 and 5.

What happens if we never call doubleCount() after count changes?

The computed signal's value stays stale because it only recalculates when accessed. So changes in base signals alone don't trigger recomputation until the computed signal is read.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of doubleCount after Step 3?

A 1

B undefined

C 2

D 3

💡 Hint

Check the 'doubleCount value' column at Step 3 in the execution_table.

At which step does the computed signal cache get invalidated?

A Step 2

B Step 4

C Step 5

D Step 3

💡 Hint

Look for when 'count' changes and cache is marked invalid in the execution_table.

If we never call doubleCount() after changing count, what happens to doubleCount's value?

A It stays the old cached value

B It resets to undefined

C It updates automatically

D It throws an error

💡 Hint

Refer to key_moments explanation about computed signals caching behavior.

Concept Snapshot

Computed signals derive values from base signals.
They cache results until base signals change.
When a base signal changes, cache invalidates.
Computed recomputes only when accessed next.
Use computed() to define derived reactive values.

Full Transcript

In Angular, computed signals let you create values derived from other signals. First, you create base signals with signal(). Then, you define a computed signal using computed() that depends on those base signals. When you access the computed signal, it runs its function and caches the result. If a base signal changes, the computed signal's cache is invalidated but it does not recompute immediately. It waits until you access it again, then it recalculates using the latest base signal values. This behavior helps optimize performance by avoiding unnecessary recalculations. The example code shows count starting at 1, doubleCount doubling it, then count changes to 3. The computed signal updates only when accessed after the change. This step-by-step trace helps beginners see how Angular's computed signals work internally.