JS: Add ellipsis if string length exceeded 50 characters

Perhaps you can do something like this:

var shortTitle = post.title.length > 50 ? post.title.substring(0,50) + "..." : post.title;

The conditional operator¹ is fairly greedy. To have it apply only to the part of your expression after the +, you need parentheses (the grouping operator):

{post.title.substring(0, 50) + (post.title.length > 50
// −−−−−−−−−−−−−−−−−−−−−−−−−−−−^
                    ? '...'
                    : '')
// −−−−−−−−−−−−−−−−−−−−−^
}

But you might consider using CSS for this, rather than JavaScript. See text-overflow: ellipsis. You'll probably need white-space: nowrap and overflow: hidden; as well:

.limit {
    max-width: 20em;
    text-overflow: ellipsis;
    white-space: nowrap;
    overflow: hidden;
}

<div>With limit:</div>
<div class="limit">Now is the time for all good men to come to the aid of their country</div>
<div>Without limit:</div>
<div>Now is the time for all good men to come to the aid of their country</div>

¹ ? : is a ternary operator (an operator accepting three operands), and for the moment it's JavaScript's only ternary operator, but there could be others in the future. Its name is the conditional operator.