Javascript / Vue3 - Mixins - Return 'null' by default

I'm building a basic 'required' form validation function. Here's the function:

JS:

export default {
    methods: {
        required(string) {
            if (!string) {
                return 'This field is required!'
            }
        }
    }
}

HTML:

<input id="username"
  v-model="credentials.username"
  type="text"
  name="username"
/>
<span>{{ required(credentials.username) }}</span>

The above works great. If I start typing in the input, the returned value goes null. If I empty the input, the returned value comes back as expected, "This field is required".

My question is, how can I return the value as null/blank to start? Expected flow is:

Returned value is null/blank to start
User starts typing, nothing changes because string.length != 0
User deletes all characters, causing string.length == 0, causing the returned value to be 'This field is required!'

Solution

One solution is to use an input-event handler (called for every new value in the input) that sets a flag to indicate the field is "dirty". Then conditionally render the validation result (the <span>) based on the flag:

Declare a data property named "dirty":

export default {
  data() {
    return {
      dirty: false,
    }
  }
}

In the template, add an input-event handler on the <input> that sets the dirty flag:
```
<input @input="dirty = true">
```

Also, conditionally render the <span> field based on dirty:

<span v-if="dirty">{{ required(credentials.username) }}</span>

demo