[рони]

OlesiaBOM,

const getQuantityPostsByAuthor = (listOfPosts, authorName) => {
    let att = 0;
    let com = 0;
    listOfPosts.forEach(({ author, comments }) => {
        att += author === authorName;
        if (comments) com += comments.filter(({ author }) => author === authorName).length
    })
    return `Post: ${att}, comments: ${com}`
};
let listOfPosts2 = [{
        id: 1,
        post: 'some post1',
        title: 'title 1',
        author: 'Ivanov',
        comments: [{
                id: 1.1,
                comment: 'some comment1',
                title: 'title 1',
                author: 'Rimus'
            },
            {
                id: 1.2,
                comment: 'some comment2',
                title: 'title 2',
                author: 'Uncle'
            }
        ]
    },
    {
        id: 2,
        post: 'some post2',
        title: 'title 2',
        author: 'Ivanov',
        comments: [{
                id: 1.1,
                comment: 'some comment1',
                title: 'title 1',
                author: 'Rimus'
            },
            {
                id: 1.2,
                comment: 'some comment2',
                title: 'title 2',
                author: 'Uncle'
            },
            {
                id: 1.3,
                comment: 'some comment3',
                title: 'title 3',
                author: 'Rimus'
            }
        ]
    },
    {
        id: 3,
        post: 'some post3',
        title: 'title 3',
        author: 'Rimus'
    },
    {
        id: 4,
        post: 'some post4',
        title: 'title 4',
        author: 'Uncle'
    }

];
console.log(getQuantityPostsByAuthor(listOfPosts2, 'Rimus'));