// Instead of this:
try {
    data = JSON.parse(data);
} catch (e) {
    output.innerHTML = "Error : Is blank or not JSON string type. Please check your data.";
    data = null;
}

// Do this:
try {
    data = JSON.parse(data);
    
    // Keep related operations in the try block
    axios.post('/post/server', data)
        .then(function (res) {
            output.className = 'container';
            output.innerHTML = res.data;
        })
        .catch(function (err) {
            output.className = 'container text-danger';
            output.innerHTML = err.message;
        });
} catch (e) {
    // Use concise, clear error messages
    output.innerHTML = "Error: empty string or invalid JSON";
    output.className = 'container text-danger';
}