Javascript Array.prototype.filter() not working

Question

I have this piece of code running on the client that filters a list of events:

if (res)
{
    eventList.filter(function(event) {

        const out = res.find(function(visibility) { return visibility.ID == event.id; }) == undefined;
        return out;
    });

    alert(eventList);
}

displayEvents(eventList);

The problem is that even when out is false the element is not filtered out.

Just for debug I tried to return false in any case and the resulting array still had all the initial elements:

eventList.filter(function(event) {

    return out;
});

What am I doing wrong here??

EDIT:

res is an array of JSON objects (containg only ID field) returned by the server, while eventList is a list of Facebook events, passed to this callback function from a Facebook API request

Answer 1

Array.prototype.filter does not change array inplace, it returns new array made of items that satisfies the provided predicate. It should look like this

var result = eventList.filter(function(event) {
    return res.find(function(visibility) { return visibility.ID == event.id; }) === undefined;
});

You don't need to declare and assign variable and then return it from function, you can simply return expression