JavaScript shows undefined fields for results from Mongoose/MongoDB

Question

JavaScript shows undefined fields for results from Mongoose/MongoDB

Why is the item logging as an object with a parameter, but when trying to access that parameter it's undefined?

My attempts so far:

console.log(item) => { title: "foo", content: "bar" }, which is fine
console.log(typeof item) => object
console.log(item.title) => "undefined"

I'll provide some context just in case it's relevant to the issue.

var TextController = function(myCollection) {
  this.myCollection = myCollection
}

TextController.prototype.list = function(req, res, next) {
  this.myCollection.find({}).exec(function(err, doc) {
    var set = new Set([])
    doc.forEach(function(item) {
      console.log(item)         // Here item shows the parameter
      console.log(item.title)   // "undefined"
      set.add(item.title)       
    })
    res.json(set.get());
  })
}

Following a suggestion, I used debugger before the problematic line to check what item actually contains via the node repl debugger. This is the result:

After discovering this, I tried console.log(item._doc.title) and it worked perfectly. So, this seems more like a mongoose-related question now than anything else.

There are similar questions, but they usually involve 'this' accessing objects or attempting to get objects outside of the function scope. In this scenario, I don't believe I'm doing either, but please correct me if I'm mistaken. Thank you.

javascript mongodb mongoose javascript-objects function-prototypes

Answer 1

Answer №1

Resolution

If you want to retrieve the fields, simply use the toObject method. Here's an example:

var itemObject = item.toObject();
console.log(itemObject.title); // "foo"

Explanation

It should be noted that the actual fields are stored within the _doc field of the document.

So why does console.log(item) display { title: "foo", content: "bar" }?

By referencing the source code of mongoose(document.js) found here, it is evident that the toString method of Document invokes the toObject method. Consequently, console.log accurately presents the fields. Refer to the excerpt from the source code below:

var inspect = require('util').inspect;

...

/**
 * Helper for console.log
 *
 * @api public
 */
Document.prototype.inspect = function(options) {
  var isPOJO = options &&
    utils.getFunctionName(options.constructor) === 'Object';
  var opts;
  if (isPOJO) {
    opts = options;
  } else if (this.schema.options.toObject) {
    opts = clone(this.schema.options.toObject);
  } else {
    opts = {};
  }
  opts.minimize = false;
  opts.retainKeyOrder = true;
  return this.toObject(opts);
};

/**
 * Helper for console.log
 *
 * @api public
 * @method toString
 */

Document.prototype.toString = function() {
  return inspect(this.inspect());
};

Answer 2

Resolution

If you want to retrieve the fields, simply use the toObject method. Here's an example:

var itemObject = item.toObject();
console.log(itemObject.title); // "foo"

Explanation

It should be noted that the actual fields are stored within the _doc field of the document.

So why does console.log(item) display { title: "foo", content: "bar" }?

By referencing the source code of mongoose(document.js) found here, it is evident that the toString method of Document invokes the toObject method. Consequently, console.log accurately presents the fields. Refer to the excerpt from the source code below:

var inspect = require('util').inspect;

...

/**
 * Helper for console.log
 *
 * @api public
 */
Document.prototype.inspect = function(options) {
  var isPOJO = options &&
    utils.getFunctionName(options.constructor) === 'Object';
  var opts;
  if (isPOJO) {
    opts = options;
  } else if (this.schema.options.toObject) {
    opts = clone(this.schema.options.toObject);
  } else {
    opts = {};
  }
  opts.minimize = false;
  opts.retainKeyOrder = true;
  return this.toObject(opts);
};

/**
 * Helper for console.log
 *
 * @api public
 * @method toString
 */

Document.prototype.toString = function() {
  return inspect(this.inspect());
};

Answer 3

Answer №2

It is important to ensure that the title field is properly defined within your schema:

var BookSchema = new mongoose.Schema({
    _id: String,
    name: String
});

Answer 4

It is important to ensure that the title field is properly defined within your schema:

var BookSchema = new mongoose.Schema({
    _id: String,
    name: String
});

Answer 5

Answer №3

Experiment with a for in loop on item to see if you can retrieve values.

for (var k in item) {
    console.log(item[k]);
}

If successful, it could indicate that your keys contain non-printable characters or something similar.

Based on your comments, it seems like item may be an instance of a primitive wrapper for a String.

For example:

var s = new String('test');
typeof s; //object
s instanceof String; //true

To test this theory, try the following:

eval('(' + item + ')').title;

Alternatively, it's possible that item is an object with a toString method that dictates its display.

EDIT: To quickly identify these issues, consider using console.dir instead of console.log. It provides an interactive list of object properties. You can also set a breakpoint and add a watch.

Answer 6

Experiment with a for in loop on item to see if you can retrieve values.

for (var k in item) {
    console.log(item[k]);
}

If successful, it could indicate that your keys contain non-printable characters or something similar.

Based on your comments, it seems like item may be an instance of a primitive wrapper for a String.

For example:

var s = new String('test');
typeof s; //object
s instanceof String; //true

To test this theory, try the following:

eval('(' + item + ')').title;

Alternatively, it's possible that item is an object with a toString method that dictates its display.

EDIT: To quickly identify these issues, consider using console.dir instead of console.log. It provides an interactive list of object properties. You can also set a breakpoint and add a watch.

Answer 7

Answer №4

Switch to using findOne() instead of find().

When you use the find() method, it will always return an array of values even if there is only one possible result. In this case, you'll need to access the desired value using item[0].

On the other hand, by utilizing the findOne() method, you are guaranteed to get just one object or none at all. This way, accessing the properties of the returned object becomes much simpler without encountering any issues.

Answer 8

Switch to using findOne() instead of find().

When you use the find() method, it will always return an array of values even if there is only one possible result. In this case, you'll need to access the desired value using item[0].

On the other hand, by utilizing the findOne() method, you are guaranteed to get just one object or none at all. This way, accessing the properties of the returned object becomes much simpler without encountering any issues.

Answer 9

Answer №5

Although this question has been asked before, I encountered the same issue and found a solution.
The mistake is most likely due to using find() instead of findOne(). By using find(), you are attempting to access an array of documents rather than just one document, which is why you end up with an array instead of a single document. Switching to findOne() will allow you to access the object as intended.

Answer 10

Although this question has been asked before, I encountered the same issue and found a solution.
The mistake is most likely due to using find() instead of findOne(). By using find(), you are attempting to access an array of documents rather than just one document, which is why you end up with an array instead of a single document. Switching to findOne() will allow you to access the object as intended.

Answer 11

Answer №6

To effectively address a problem like this, you can utilize the doc.toObject() method in the following manner:

doc.toObject({ getters: true })

Additional options to consider are:

getters: activates all getters (path and virtual getters)
virtuals: activates virtual getters (can override getters option)
minimize: eliminates empty objects (defaults to true)
transform: a function to apply when transforming the document before returning
depopulate: replaces populated paths with their original references (defaults to false)
versionKey: determines whether to include the version key (defaults to true)

For example, you could implement it like this:

Model.findOne().exec((err, doc) => {
   if (!err) {
      doc.toObject({ getters: true })
      console.log('doc _id:', doc._id) // or title
   }
})

With these adjustments, your code should now function correctly.

Answer 12

To effectively address a problem like this, you can utilize the doc.toObject() method in the following manner:

doc.toObject({ getters: true })

Additional options to consider are:

getters: activates all getters (path and virtual getters)
virtuals: activates virtual getters (can override getters option)
minimize: eliminates empty objects (defaults to true)
transform: a function to apply when transforming the document before returning
depopulate: replaces populated paths with their original references (defaults to false)
versionKey: determines whether to include the version key (defaults to true)

For example, you could implement it like this:

Model.findOne().exec((err, doc) => {
   if (!err) {
      doc.toObject({ getters: true })
      console.log('doc _id:', doc._id) // or title
   }
})

With these adjustments, your code should now function correctly.

Answer 13

Answer №7

Are you careful to avoid including white space or special characters in the 'title' attribute? These should be defined if identifiers are placed within the object or map declaration. Take this example:

var challenge = {
    'title': 'Example',
    'description': 'Illustration'
};

While this may cause console.log(item) to appear as expected, it could lead to an issue of returning undefined if you try to access the title property without the proper spacing.

Answer 14

Are you careful to avoid including white space or special characters in the 'title' attribute? These should be defined if identifiers are placed within the object or map declaration. Take this example:

var challenge = {
    'title': 'Example',
    'description': 'Illustration'
};

While this may cause console.log(item) to appear as expected, it could lead to an issue of returning undefined if you try to access the title property without the proper spacing.

Answer 15

Answer №8

In my experience, the 'find' method in MongoDB usually returns an array of Documents. I tested this out myself and successfully printed out the title of each document.

for (let j = 0; j < items.length; j++) {
   console.log("loop number " + j);
   console.log('Document ID:' + items[j]._id);
   console.log(items[j].title);
}

Answer 16

In my experience, the 'find' method in MongoDB usually returns an array of Documents. I tested this out myself and successfully printed out the title of each document.

for (let j = 0; j < items.length; j++) {
   console.log("loop number " + j);
   console.log('Document ID:' + items[j]._id);
   console.log(items[j].title);
}

Answer 17

Answer №9

For those who simply seek information without the added benefits of mongoose, such as speed, you can opt to use .lean() in your query. This will retrieve the information more quickly and allow you to directly use it as an object.

https://mongoosejs.com/docs/api.html#query_Query-lean

According to the documentation, this method is ideal for read-only scenarios.

Answer 18

For those who simply seek information without the added benefits of mongoose, such as speed, you can opt to use .lean() in your query. This will retrieve the information more quickly and allow you to directly use it as an object.

https://mongoosejs.com/docs/api.html#query_Query-lean

According to the documentation, this method is ideal for read-only scenarios.

Answer 19

Answer №10

Have you properly set up your object?

function NewObject()
{
    this.Name = "";
    this.Description = "";
}

var newObj1 = new NewObject();

If you neglect to initialize or leave a field empty, you may receive an undefined value.

Answer 20

Have you properly set up your object?

function NewObject()
{
    this.Name = "";
    this.Description = "";
}

var newObj1 = new NewObject();

If you neglect to initialize or leave a field empty, you may receive an undefined value.

Answer 21

Answer №11

Always remember to include the .lean() method in your queries, like this:

const product = await Product.findById("76483927").lean()

Answer 22

Always remember to include the .lean() method in your queries, like this:

const product = await Product.findById("76483927").lean()

Answer 23

Answer №12

To retrieve elements from the returned array of objects in JavaScript, you can use indexing. For example, to access the title property of the first object in the array, you would write doc[0].title.

Answer 24

To retrieve elements from the returned array of objects in JavaScript, you can use indexing. For example, to access the title property of the first object in the array, you would write doc[0].title.

JavaScript shows undefined fields for results from Mongoose/MongoDB

Answer №1

Resolution

Explanation

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Answer №7

Answer №8

Answer №9

Answer №10

Answer №11

Answer №12

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