Have you ever noticed that Vue re-executes any method you are using anywhere in your HTML code between the curly braces or with

v-bind

or

v-Html

or with any non-event bound method if something is changed on the screen?

No? see the running example below.

It's true that Vue re-executes methods(non-bound) if anything on the screen changes, and that's why it's called a reactive framework, right. But suppose if you have hundred's of method and you change something on the screen and Vue starts executing these hundred's methods just imagine the performance of the app.

let's see this example :

<template>
  <div id="app">
    <Div>
      <span>Counter :{{ counter }}</span> <br />
      <button @click='counter++'>Increase</button> </t>
      <button @click='counter--'>Decrease</button>
    </Div>
    <p>{{ currentTime() }}</p>
  </div>
</template>

<script>
import VueExample from './components/HelloWorld.vue';

export default {
  data() {
        return {
            counter: 0,
        };
    },
  methods: {
    currentTime() {
      var date = new Date();
      console.log("in another method")
      return date;
    },
  },
};
</script>

In this example, there are two buttons that increases and decreases the counter upon clicking, and there is also a this code for displaying current time

{{ <p>{{ currentTime() }}</p>}}

When you click on any of the button, it executes the currentTime() method which we can verify by either looking at the console log or just by checking the current time at the UI . Though counter has nothing to do with the currentTime() method, but still it gets executed every time you increase or decrease the counter.

Why?

The reason is that Vue doesn't know whether the counter attribute is part of currentTime() method or not, in other words, we can say Vue is not aware of whether currentTime() has any dependency on the counter attribute or not, so it executes it every time.

Check the running example here

So, how do we stop it?

Well, the solution to this issue is the "Computed Property."

A computed property is similar to other properties in Vue, but in a computed property, Vue is aware of its dependency, and it only executes it when any of it's dependencies changes.

So, lets fix the above code.

For this, we just need to move the
currentTime()
from method section to computed section

  <div id="app">
    <Div>
      <span>Counter :{{ counter }}</span> <br />
      <button @click='counter++'>Increase</button> </t>
      <button @click='counter--'>Decrease</button>
    </Div>
    <p>{{ currentTime }}</p>
  </div>
</template>

<script>
import VueExample from './components/HelloWorld.vue';

export default {
  data() {
        return {
            counter: 0,
        };
    },
  computed:{
    currentTime() {
      var date = new Date();
      console.log("in another method")
      return date;
    },
  },
  methods: {
  },
};
</script>

There is another approach to solve this using Watcher, which I "ll discuss in my upcoming articles.