Adding comments to images
Your link has an assignment description, which I assume you have to follow, and I'll do so too. It says to use ES6 and class, but you have only used class for one of these, and even then you still use .prototype. You also shouldn't be using var, use let and const instead.

Comment class

Use class syntax. Save the image object instead of the imageId. Make the findImage function. commentEl should return a string, not a jQuery object.

class Comment {
    constructor(imageId, text) {
        this.id = Comment.all.length;
        this.image = this.findImage(imageId);
        this.text = text;
        Comment.all.push(this);
    }

    findImage(imageId) {
        const image =  Image.all.find(image =&gt; image.id === Number(imageId));
        image.comments.push(this);
        return image;
    }

    commentEl() {
        return `&lt;li id='comment-${this.id}'&gt;${this.text}&lt;/li&gt;`;
    }
}

Comment.all = [];


CommentsController class

Use class syntax. Remove $addCommentForm since it's never used. addCommentFormListener doesn't add a listener, it is a listener (or handler, if you will). init is the one actually adding the listener. render can be simplified, since part of it is being done elsewhere. It is also being used like a static method.

class CommentsController {
    constructor() {
        this.addCommentFormListener();
    }

    addCommentFormListener() {
        $(".add-comment").each((index, element) =&gt; {
            element.addEventListener("submit", commentFormHandler);
        });
    }

    static render(commentObject) {
        const commentList = $(`#comments-${commentObject.image.id}`);
        commentList.append(commentObject.commentEl());
    }
}

// This could also be part of the class
function commentFormHandler(e) {
    e.preventDefault();
    const image = $(this).closest(".image");
    const imageId = Number(image.find("ul").attr("data-id"));
    const commentForm = $(this).find('input[name="comment-description"]')
    const commentContents = commentForm.val();
    if (commentContents.length) {
        const newComment = new Comment(imageId, commentContents);
        CommentsController.render(newComment);
        commentForm.val("");
    }
}


I'm not a fan of the way you're getting the imageId and comment. It's tied too much to the DOM and a bit confusing. But that's beyond this question.