$in Operator in MongoDB usecases

Show a feed of posts made only by users you are following like X’s “Following” section

const followingIds = ['user1', 'user2', 'user3'];
//Get posts from your following list in the last 24 hours and sort them in descending order
const feedPosts = await Post.find({
  author: { $in: followingIds },
  createdAt: { $gte: new Date(Date.now() - 24 * 60 * 60 * 1000) } 
}).sort({ createdAt: -1 });

Select multiple filters of the same category

Say you have a state that stores multiple filters of the same category and you want to send this in your REST API

In Express, when multiple query parameters have the same name (e.g., ?filter=shirt&filter=pants), the framework handles them by parsing the repeated parameters into an array. This means if a URL includes the query string ?filter=shirt&filter=pants, Express will parse req.query.filter as an array: ['shirt', 'pants']

However, If you send only one filter (e.g., ?filter=shirt), Express will parse req.query.filter as a string instead of an array.

http://127.0.0.1:4000/api?filter=red&filter=black&filter=grey

//Frontend

const url = "http://127.0.0.1:4000/api?brand=nike"
const queryString = selectedFilters.map(filter => `filter=${encodeURIComponent(filter)}`).join('&');
  fetch(`${url}${queryString}`)
        .then(res => res.json())
        .then(data => {
            setData(data);
       }

//Backend

const getData = async (req, res) => {

        // Extract the filter parameter from the query string
        let { filter=[], brand } = req.query;

        if(typeof filter === "string"){
            filter = [filter]
        }

        let query = {brand: brand};
        if (filter && filter.length > 0) {
            query = { ...query, type: { $in: filter }};
        }


        const clothes = await clothesModel.find(query).exec();
 }