post = [
{postTitle : 'Title1', key = 'key1'},
{postTitle : 'Title2', key = 'key2'}
]

comment = [
{key : 'key1', c: "comment1"},
{key : 'key1', c: "comment2"},
{key : 'key2', c: "commentA"},
{key : 'key2', c: "commentB"},
{key : 'key2', c: "commentC"}

]

newPost = [
{
postTitle : 'Title1', key = 'key1', comment: {"comment1", "comment2"}
},
{
postTitle : 'Title2', key = 'key2', comment: {"commentA", "commentB", "commentC"}
}
]

for (var i = 0; i < post.length; ++i) {
    for (var j = 0; j < comment.length; j++) {
        if (post[i].key === comment[j].key) {
            post[i].comment = [];
            post[i].comment.push([comment[j].c]);
        }
        else {
        console.log('no comment to add');        }
    }
}