The problem here is whenever the page loads, and you call getItem() you reset localStorage.count = 0 since it automatically initializes to 0.

Instead, do something like this:

const app = Vue.createApp({
  data() {
    return {
      counter: localStorage.getItem('count') || 0,
    };
  },
  watch: {
    counter() {
      localStorage.setItem('count', this.counter);
    }

  },
  methods: {
    getItem() {
      return localStorage.getItem('count');
    },
  },
}).mount('#app');

This way, you're setting the initial value to the value from storage; and watching that value for a change and then updating localStorage to reflect the correct value stored across the window.

This solution should work for you:

Here is an improved version:

In this solution, I used a ternary operator that asks first if the counter property exists in the local storage if not then the counter will initialize with the value 0, if the counter is there will get the value and parse that value to int value as all value in the local store save as string.

So if you sum the value a string will do the following:

step: 1

counter = '0';

step: 2

counter++

in this step, the counter will parse to an Integer but still be 0;

Instead, if you do what I code

step: 1

counter = parseInt('0');

step: 2

counter++, the counter at the first time will be 1

As this code is for vuejs version 3 the snippet will not work

const app = Vue.createApp({
  data() {
    return {
      counter: localStorage.getItem('count') ? parseInt(localStorage.getItem('count')) : 0,
    };
  },
  watch: {
    counter(newQuestion, oldQuestion){
      localStorage.setItem('count', newQuestion)
    }
  },
}).mount('#app');

<div id="app">
   <h2>{{counter}}</h2>
   <button @click="counter--">-</button>
   <button @click="counter++">+</button>
</div>

Use computed properties. So you dont need methods, watchers or data

const app = Vue.createApp({
  computed: {
     counter: {
        get() {
           return localStorage.getItem('count') || 0
        },
        set(val) {
           localStorage.setItem("count", val)
        }
     }
  },
}).mount('#app');