// <ul id="r"></ul>
fetch('https://iwb.jp/wp-json/wp/v2/posts')
.then(response => response.json())
.then(data => {
const r = document.getElementById('r')
const html = data.reduce((a, c) => {
a.push(c.title.rendered)
return a
}, [])
r.innerHTML = '<li>' + html.join('</li><li>') + '</li>'
});