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>
 

Not sure about the correct method. I share what I did on my project.

1. in rollup.config.js

   import replace from "@rollup/plugin-replace";
   const production = !process.env.ROLLUP_WATCH;
   

2. inside plugins:[ ] block add this

   replace({
       isProduction: production,
    }),
   

   rollup.config.js will look like this:

   plugins: [
     replace({
       isProduction: production,
     }),
     svelte({
       // options
     }),
   ]
   

3. Then use isProduction inside components.

   if (!isProduction){ console.log('Developement Mode'); }