Missing property at client side Nodejs

The problem is that item is a MongooseDocument, not a plain javascript object.

There are multiple ways to achieve what you want.

Documents returned from queries with the lean option enabled are plain javascript objects, not MongooseDocuments

Model.findOne().lean().exec((err, item) => {    item.mention = [{ id: 1 }];    res.json(item);});

This option will increase the performance since you ignore the overhead of creating the Mongoose Document

2) Using: .set()

item.set('mention', [{ id: 1}], { strict: false });

res.json({    status: 1,    message: 'success',    data: { mention: [{ id: 5 }], ...item }})

4) Using Object.assign as @Alexis Facques explained.

node.js object mongoose server

Taking advantage of Object.assign() should resolve your problem too.

res.json({    status: 1,    message: 'success',    data: Object.assign({ mention: [{id: 1...}] },item)})

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