Persisting store state in Vue - Step-by-Step Execution

Concept Flow - Persisting store state

App starts

↓

Load store state from storage?

No→Use default state

Yes↓

Initialize store with loaded state

↓

User interacts -> store state changes

↓

Save updated state to storage

↩Back to User interacts

When the app starts, it tries to load saved state from storage. If found, it initializes the store with it. When the user changes state, the new state is saved back to storage.

Execution Sample

Vue

import { reactive, watch } from 'vue'
const store = reactive({ count: 0 })
const saved = localStorage.getItem('store')
if (saved) store.count = JSON.parse(saved).count
watch(store, () => localStorage.setItem('store', JSON.stringify(store)))

This code loads a saved count from localStorage into a reactive store and saves changes back automatically.

Execution Table

Step	Action	Store State Before	Operation	Store State After	Storage Interaction
1	App starts	{ count: 0 }	Check localStorage	{ count: 0 }	localStorage.getItem('store') returns '{"count":5}'
2	Load saved state	{ count: 0 }	Parse and assign	{ count: 5 }	No change
3	User clicks increment	{ count: 5 }	store.count++	{ count: 6 }	No change yet
4	Watch triggers	{ count: 6 }	Save to localStorage	{ count: 6 }	localStorage.setItem('store', '{"count":6}')
5	User clicks increment again	{ count: 6 }	store.count++	{ count: 7 }	No change yet
6	Watch triggers	{ count: 7 }	Save to localStorage	{ count: 7 }	localStorage.setItem('store', '{"count":7}')
7	App reloads	{ count: 0 }	Check localStorage	{ count: 0 }	localStorage.getItem('store') returns '{"count":7}'
8	Load saved state	{ count: 0 }	Parse and assign	{ count: 7 }	No change
9	Exit	{ count: 7 }	No more actions	{ count: 7 }	State persisted

💡 Execution stops because no more user actions occur and state is persisted.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
store.count	0	5	6	7	7	7
localStorage 'store'	{"count":5}	{"count":5}	{"count":6}	{"count":7}	{"count":7}	{"count":7}

Key Moments - 3 Insights

Why does the store state update after loading from localStorage?

Because at step 2, the saved JSON string is parsed and assigned to the reactive store, updating store.count from 0 to 5 as shown in the execution_table.

When does the localStorage get updated with the new store state?

At steps 4 and 6, the watch function triggers after store.count changes, saving the updated state to localStorage as shown in the Storage Interaction column.

What happens if localStorage is empty when the app starts?

At step 1, localStorage.getItem returns null, so the store keeps its default state (count: 0) as shown in the execution_table row 1.

Visual Quiz - 3 Questions

Look at the execution table, what is store.count after step 4?

A 6

B 5

C 7

D 0

💡 Hint

Check the 'Store State After' column at step 4 in the execution_table.

At which step does localStorage first get updated with the new store state?

A Step 6

B Step 2

C Step 4

D Step 8

💡 Hint

Look at the 'Storage Interaction' column to see when localStorage.setItem is first called.

If localStorage was empty at app start, what would store.count be after step 2?

A null

B 0

C 5

D undefined

💡 Hint

Refer to the 'Store State After' column at step 2 and the explanation in key_moments about empty localStorage.

Concept Snapshot

Persisting store state in Vue:
- On app start, load saved state from localStorage if exists.
- Initialize reactive store with loaded or default state.
- Use watch to save store changes back to localStorage.
- This keeps state across page reloads.
- localStorage stores strings, so use JSON.stringify/parse.

Full Transcript

This visual execution shows how a Vue reactive store can keep its state saved in localStorage. When the app starts, it checks localStorage for saved data. If found, it loads and parses it to update the store's state. When the user changes the store (like incrementing a count), a watch function triggers and saves the new state back to localStorage as a JSON string. This way, the state persists even if the page reloads. The execution table traces each step, showing store values and storage interactions. The variable tracker highlights how store.count and localStorage content change over time. Key moments clarify common confusions like when loading and saving happen. The quiz tests understanding by asking about specific steps and values. This method helps beginners see exactly how persisting store state works in Vue with localStorage.