<p><input type="date" id="date" value=""></p>
<p><button id="btn">本日の日付を反映する</button></p>
const btn = document.getElementById('btn')
const date = document.getElementById('date')
btn.addEventListener('click', () => {
const now = new Date()
const formatDate = `${now.getFullYear()}-${('0' + (now.getMonth() + 1)).slice(-2)}-${('0' + now.getDate()).slice(-2)}`
console.log({formatDate})
date.value = formatDate
})