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:

```svelte
<script>
    import {onMount} from 'svelte';

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

```