How to prevent duplicate items in array?

Question

I have two arrays, i want to push some items from arr1 to arr2 preventing duplicates because i've already added some items to arr2 that are also in arr1. i am doing this in react js

                arr1.forEach(q1 => {
                    arr2.forEach(q2 => {
                        if (q1.lessonId !== q2.lessonId) {
                            if (q2.obtainedMarks >= q2.passingMarks) {
                                User.findByIdAndUpdate(req.body.userId, { $push: { arr1: { $each: 
                                    [q2._id] } } }, (err, doc) => {

                                })
                            }
                        }
                    })
                });

Answer 1

You can use a set instead of an array. They have the built in feature of preventing duplicate entries. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set