How to differentiate between Svelte dev mode and build mode?

The dev mode using npm run dev, the release mode using npm build

How could i know that it's currently built on dev mode or not in the code, for example:

<script>    import {onMount} from 'svelte';    onMount(function(){        if(DEVMODE) { // --> what's the correct one?            console.log('this is x.svelte');        }    })</script>