Seems okay to me to be honest. What you could do is replace the "50" with a variable in your js file.
var MESSAGE_MAX_LENGTH= 50;
And re position a bit the way you address the function
function getShortMessages(messages) {
return messages
.filter( (messageObject) => {
return messageObject.message.length < MESSAGE_MAX_LENGTH
})
.map( (messageObject) => {
return messageObject.message
});
}
Also I find that when addressing an array of messages and running through the filter function, it is better to not call the object messages but item or messageObject
moreover, object in the map function is a bit ominous, call it messageObject again for example so you know what you are using specifically