mongoose — check if ObjectId exists in an array

Following is an example model:

UserModel == {
    name: String,
    friends: [ObjectId],

friends corresponds to a list of id of objects of some other model for example AboutModel.

AboutModel == {
    name: String,

User.findOne({name: 'Alpha'}, function(error, user){
    About.find({}, function(error, abouts){ // consider abouts are all unique in this case
        var doStuff = function(index){
            if (!( in user.friends)){
            if (index + 1 < abouts.length){
                doStuff(index + 1)
        doStuff(0) // recursively...

In this case, the condition ' in user.friends` seems to be always false. How? Is this to do with the type of ObjectId or the way it is saved?

Note: ObjectId is short for Schema.ObjectId; I don't know if that's an issue in itself.


If is a string representation of an ObjectID and user.friends is an array of ObjectIDs, you can check if is in the array using Array#some:

var isInArray = user.friends.some(function (friend) {
    return friend.equals(;

The some call will iterate over the user.friends array, calling equals on each one to see if it matches and stop as soon as it finds a match. If it finds a match it returns true, otherwise false.

You can't use something simpler like indexOf because you want to compare the ObjectIDs by value, not by reference.

I use lo-dash and do something like that :

var id_to_found = '...';
var index = _.find(array, function(ch) {
     return ch == id_to_found ;
if ( index!=undefined ) {
} else {

I believe this is a javascript question rather than a Node.js/Mongoose question - so it really doesn't belong in the way it is currently now.

Also, the issue with in user.friends is that the object pointed to by and objects in user.friends are different; and I believe in checks for the equality of the object.

Anyway, the answer is available on stack overflow to check where an element exists in an array —

user.friends.indexOf( > -1

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