Error: Templates should only be responsible for mapping the state to the UI. Avoid placing tags with side-effects in your templates, such as

You can use script tag in this way and it will work fine.
I was facing the same problem when I used <script></script> tag without specifying its type.
After using the type attribute Vue did not warn me for critical error:

<script type="application/javascript"> 
    // your code
</script>