Laravel blade.php use JavaScript compiled by mix to app.js

I'd like to use a JavaScript function in my blade.php file. I created the xyz.js file with a very simple function

function picture(soemthing){
    document.getElementById('idXYZ').src = "example.com";
}

Then I added this file to the webpack.mix.js config:

mix.js(['resources/assets/js/app.js', 'resources/assets/js/xyz.js'], 'public/js')
.sass('resources/assets/sass/app.scss', 'public/css');

and run: npm run dev. So far so good, npm compiled my script and the picture function was included in the app.js file. Now I'd like to add a form in blade.php and call the picture function on click. I created the following blade.php file:

@extends('layouts.app')
@section('content')
<div class="container">
    <form>
        Enter url: <input type="text" id="someID">
        <input type="button" onclick="picture($('#someID').val())" value="Click"/>
    </form>
</div>
<script src=""></script>
@endsection

And here I got really lost (btw: I was always the backend guy). On page reload browser displays

[Vue warn]: Error compiling template: Templates should only be responsible for mapping the state to the UI. Avoid placing tags with side-effects in your templates, such as , as they will not be parsed.

And when I click the button I have

Error new:76 Uncaught ReferenceError: xyz is not defined

I understand that the first errors says that the app.js file was not loaded and that's the reason why picture is not found. How should I include app.js or any other compiled by mix JavaScript file? Is it really the tag causing this problem?

via orim