How can I limit the post title to 70 characters with the last post's script I use on blogger?

The post title is sometimes more than 70 characters, which distorts the image.

    let postCount = 5;
    
    function funcrct(JSON) {
        const POSTS = JSON.feed.entry;
        let postTitle, postUrl = "";
        let rpc = document.querySelector(".postList");
        for (let i = 0; i < postCount; i++) {
            POSTS[i].link.forEach((el, i) => {
                if (el.rel === "alternate") {
                    postTitle = el.title;
if (postTitle.length >= 70) postTitle.subString(0,69)
                    postUrl = el.href;
                }
            })
    
            let thumbnail = POSTS[i].content.$t.match(/(http)?s?:?(\/\/[^"']*\.(?:webp))/)[0];
    
            rpc.innerHTML += `<li>
    <a href="${postUrl}"><figure>       
    <img src="${thumbnail}" alt="${postTitle}"/>
    <figcaption>
    ${postTitle}</figcaption></figure></a>
    </li>`;
        }
    }
    
    var url = document.location.origin;
    var fsrc = document.createElement("script");
    fsrc.src = `${url}/feeds/posts/default?alt=json-in-script&start-index=1&max-results=${postCount}&callback=funcrct`;
    
    
    document.body.appendChild(fsrc);