i have a collection of Ebooks data in mongodb like
{ \"_id\" : ObjectId(\"58b56fe19585b10cd42981d8\"), \"cover_path\" : \"D:\\\\Ebooks\\\\uploads\\\\eb
Try wrapping your query in curlies, Mongoose expects an object as the query.
app.get('/ebook?search=',function(req,res){ var search_key = req.param('search'); Ebook.find({title: search_key}) .then(ebooks => res.json(ebooks)) .catch(err => res.status(404).json({ success: false })); });