Use the following to hash your script and out the string needed for SRI.

echo -n "sha256-" && openssl dgst -sha256 -binary ./path/to/file.js | openssl base64 -A; echo

Use the following to load your script

<script defer src="./path/to/file.js" integrity="sha256-<hash>"></script>

Using scripts from another domain?

If using scripts from another domain such as a CDN you’ll need to include crossorigin="anonymous" or the integrity check won’t be performed by the browser.

Use the following to load your script:

<script defer src="/assets/js/template-web.min.js" integrity="sha256-0sL5IkMJDqqdc9pElxsYupSMQjKt2+Ua6dCMPoDrwGg=" crossorigin="anonymous"></script>

Further reading

Check out the Wikipedia article and SRI Hash Generator