<textarea></textarea>
<br/>
<button>Save to file</button>

const ta = document.querySelector('textarea');
const btn = document.querySelector('button');

btn.addEventListener('click', () => {
    const url = URL.createObjectURL(new Blob([ta.value]), 'text/plain');
    const link = document.createElement('a');
    link.setAttribute('download', true);
    link.setAttribute('href', url);
    link.click();
})