load an external JS library in Svelte

// document.js

export function loadScript(src) {
    return new Promise((resolve, reject) => {
        const script = document.createElement('script');
        script.src = src;

        document.body.appendChild(script);

        script.addEventListener('load', () => resolve(script));
        script.addEventListener('error', () => reject(script));
    });
}

// index.svelte
<script>
    import { onMount } from 'svelte';
    import { loadScript } from './document.js';

    onMount(async () => {
      await loadScript('your-external-script.js');
      console.log('script loaded successfully!');
    });
</script>