In JavaScript, if you check for the existence of a key in an object, it

Question

In JavaScript, if you check for the existence of a key in an object, it

Recently, I ran into an issue with an undefined error when trying to access a value in a JavaScript object key. I retrieved arrays of objects using the mongoose.find().exec() callback and then checked each object for a specific key.

Here is an example object:

{"promo_id":"5af1c07d5542fc9307847a6a",
  "__v":0,
   "description":"Hello", 
 "available_count":0}

When I tested
```
obj.hasOwnProperty('available_count')
```
, it unexpectedly returned false.
Using lodash's _.isObject() method on the object returned true.
Ultimately, I had to resort to JSON.parse(JSON.stringify(obj)) to resolve the issue.

I am perplexed by this strange behavior and would appreciate any insights on why this is happening.

let o = {"promo_id":"5af1c07d5542fc9307847a6a",
  "__v":0,
   "description":"Hello", 
 "available_count":0}
 
 let result = o.hasOwnProperty('available_count');
 
 console.log(result)

javascript

Answer 1

Answer №1

According to the documentation provided by Mongoose, when you use query.find(), it returns an array of documents. These Mongoose documents are unique in that they do not directly correspond to JavaScript literal objects:

Mongoose documents maintain a one-to-one mapping with the documents stored in MongoDB, with each document representing an instance of its Model.

http://mongoosejs.com/docs/documents.html

If you refer to the Document API, you will find a toObject() method. It is recommended to try using this method on the first item in the returned array.

You may be wondering why JSON.parse(JSON.stringify(obj)) works for you. This behavior can be attributed to the fact that the MDN documentation on JSON.stringify mentions the use of a toJSON method during stringification. If the object being stringified has implemented this method, it customizes the stringification process. In the case of Mongoose documents, as indicated here, they indeed implement this method, leading to correct stringification into a JavaScript Object Literal which can then be parsed by JSON.parse.

Answer 2

According to the documentation provided by Mongoose, when you use query.find(), it returns an array of documents. These Mongoose documents are unique in that they do not directly correspond to JavaScript literal objects:

Mongoose documents maintain a one-to-one mapping with the documents stored in MongoDB, with each document representing an instance of its Model.

http://mongoosejs.com/docs/documents.html

If you refer to the Document API, you will find a toObject() method. It is recommended to try using this method on the first item in the returned array.

You may be wondering why JSON.parse(JSON.stringify(obj)) works for you. This behavior can be attributed to the fact that the MDN documentation on JSON.stringify mentions the use of a toJSON method during stringification. If the object being stringified has implemented this method, it customizes the stringification process. In the case of Mongoose documents, as indicated here, they indeed implement this method, leading to correct stringification into a JavaScript Object Literal which can then be parsed by JSON.parse.

In JavaScript, if you check for the existence of a key in an object, it

Answer №1

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