Develop Reference

Javascript - Access nested elements of JSON object

So I have a series of 4 JSON objects with nested data inside each of them. Each of these objects are stored in an array called classes. Here is an example of how one of the class objects is formatted:
let class_A = {
professor: "Joey Smith",
numberStudents: 25,
courseCode: "COMS 2360",
seating: {
"FirstRow": {
0: {
firstName: "Sarah",
collegeMajor: "English",
},
1: {
firstName: "Bob",
collegeMajor: "Computer Engineering",
},
2: {
firstName: "Dylan",
collegeMajor: "Mathematics",
}
},
"SecondRow": {
3: {
firstName: "Molly",
collegeMajor: "Music"
}
}
}
};
I'm struggling to figure out how to access the very last fields within each class object (firstName and collegeMajor). The furthest I was able to get was the indexes beneath each row number.
let classes = [class_A, class_B, class_C, class_D];
let classesAvailable = document.getElementById('classes');
let class = classes[classesAvailable.value];
for(rowNum in class.seating){
for(index in class.seating[rowNum]){
console.log(index);
//console.log(class.seating[rowNum[index]].firstName);
}
}
So in this example, console.log(index) prints out:
0
1
2
3
but I'm unable to print the first name and college major of each student in each row. I was trying to follow a similar logic and do console.log(class.seating[rowNum[index]].firstName) but I get the error:
Cannot read properties of undefined (reading 'firstName')
Was wondering if anyone knows what's wrong with my logic here?

console.log(class.seating[rowNum][index])

Javascript object as a type of mini database?

Is it possible to use a JavaScript object as a type of mini database? I often find myself needing a kind of database structure when I'm coding in JS but it feels like overkill to use an actual database like MySQL (or similar).
As an example, let's say I need to structure this data as a JS object:
Object idea: Stuff to sell
Items to sell: The junk in the garage
Object structure: List all items including item name, item condition, and item value
In order to make this into a JS object I would maybe write:
var stuffToSell = {};
Then my first item would maybe look like:
var stuffToSell = {
item : "Coffee Maker",
condition : "Good",
price : 5
};
Now to me this seems like I'm on the right track, until I come to add another item and I end up having to use the properties item, condition, and price again in the same JS object — which feels wrong? — or is it?? At this point my brain keeps shouting the word "ARRAY!" at me but I just can't see how I can use an array inside the object, or an object inside an array to achieve what I want.
My end goal (in this simplified example) is to be able to then use object-oriented syntax to be able to access certain items and find out specific information about the item such as price, condition etc. For example if I want to know the price of the "coffee maker" I would like to write something like:
stuffToSell["coffee maker"].price
...and then the result above should be 5.
I feel like I'm on the right track but I think I'm missing the array part? Could someone please tell me what I'm missing or maybe what I'm doing completely wrong! And also if it is wrong to have duplicate property names in the same JS object? For example, is it okay to have:
var stuffToSell = {
item : "Coffee Maker",
price : 5,
item : "Mountain Bike",
price : 10,
item : "26 inch TV",
price : 15
};
...it seems wrong because then how does JS know which price goes with which item??
Thanks in advance :)

You're definitely on the right track!
A lot of people will refer to what you're talking about as a hash.
Here's my suggested structure for you:
var store = {
coffee_maker: {
id: 'coffee_maker',
description: "The last coffee maker you'll ever need!",
price: 5,
},
mountain_bike: {
id: 'mountain_bike',
description: 'The fastest mountain bike around!',
price: 10,
},
tv: {
id: 'tv',
description: 'A big 26 inch TV',
price: 15,
},
}
Having a structure like that will let you do this:
store.mountain_bike.price // gives me 10
Need an array instead, say for filtering or looping over?
Object.keys gives you an Array of all the object's keys in the store ['coffee_maker', 'mountain_bike', 'tv']
// Now we just have an array of objects
// [{id: 'coffee_maker', price: 5}, {id: 'mountain_bike', price: 10} ...etc]
var arr = Object.keys(store).map(el => store[el])
Need to just filter for items that are less than 10?
This will give us an array of products less than 10:
// gives us [{id: 'coffee_maker', price: 5}]
var productsUnder10 = arr.filter(el => el.price < 10)
These techniques can be chained:
var productsOver10 = Object.keys(store)
.map(el => store[el])
.filter(el => el.price > 10)
Need to add a product?
store['new_product'] = {
id: 'new_product',
description: 'The Newest Product',
price: 9000,
}
Here's another way, which would be good to start getting used to.
This is a 'safe' way to update the store, read up on immutability in javascript to learn about it
store = Object.assign({}, store, {
'new_product': {
id: 'new_product',
description: 'The Newest Product',
price: 9000,
}
})
...and another way, that you should also read up on and start using:
This is the object spread operator, basically just an easier way to work with immutable structures
store = {
...store,
'new_product': {
id: 'new_product',
description: 'The Newest Product',
price: 9000,
}
}
Resources
JavaScript Arrow Functions
Object and Array Spread Syntax
Immutable Javascript using ES6 and beyond

You can actually use json or create an array of objects.If using a separate file to store the objects, first load the file. Use array filter method to get an new array which matches the filter condition , like you want to get the item with id 1. This will return an array of objects.
var dict = [{
'id': 1,
'name': 'coffee-mug',
'price': 60
},
{
'id': 2,
'name': 'pen',
'price': 2
}
]
function getItemPrice(itemId) {
var getItem = dict.filter(function(item) {
return item.id === itemId
});
return getItem[0].price;
}
console.log(getItemPrice(1))

JSON objects don't support repeated keys, so you need to set unique keys.
Put an id as your key to group your items:
var stuffToSell = {
'1': {
item: "Coffee Maker",
price: 5
},
'2': {
item: "Mountain Bike",
price: 10
}
.
.
.
}
Now you can access the item's price very fast.
Look at this code snippet (Known Ids)
var stuffToSell = {
'1': {
item: "Coffee Maker",
price: 5
},
'2': {
item: "Mountain Bike",
price: 10
},
'3': {
item: "26 inch TV",
price: 15
}
};
let getPrice = (id) => stuffToSell[id].price;
console.log(getPrice('1'));
See? the access to your items it's fast and your code follows a readable structure.
Look at this code snippet (Item's name as key)
var stuffToSell = {
'Coffee Maker': {
price: 5
},
'Mountain Bike': {
price: 10
},
'26 inch TV': {
price: 15
}
};
let getPrice = (id) => stuffToSell[id].price;
console.log(getPrice('Coffee Maker'));
Look at this code snippet (Item's name: price)
var stuffToSell = {
'Coffee Maker': 5,
'Mountain Bike': 10,
'26 inch TV': 15
};
let getPrice = (id) => stuffToSell[id];
console.log(getPrice('Coffee Maker'));

Remove Attribute names from Json string

Hi lets say I have a JSON string which represent a record in a grid containing 3 columns Id, Name, Status. I'm currently writing some JavaScript logic where you can filter the rows of data by typing some text in the text box. The filter will be applied to data in all columns. So if I type "James" Record 1 below will be returned an if I type None Record 1 and 2 will be returned. The problem is if I type Id, Name, or Status which is not the data but the attribute names, all records are always returned.
Record 1
{
Id: 1,
Name: "James",
Status: "None"
}
Record 2
{
Id: 2,
Name: "Paul",
Status: "None"
}
How can I modify a JSON string so that
{ Id: 2, Name: "Paul", Status: "None"}
will become
{ 2, "Paul", "None"}

Your question is a bit unclear (and I'm afraid Matthias' edit made that matter worse).
{ Id: 1, Name: "James", Status: "None" } is not a valid JSON string, but it is a valid Javascript object. JSON strings need to have their values within quotes.
If you are indeed dealing with a JSON string, with quoted properties, and you simply want the output you've requested, you could do something like this:
var person = '{ "Id": 1, "Name": "James", "Status": "None" }';
person = person.replace(/\s*"[^"]+"\s*:/g,"");
// > person = '{ 1, "James", "None" }'
If you are dealing with a Javascript object, a simple way of getting the values without the property names would be to do something like this:
var person = { Id: 1, Name: "James", Status: "None" };
person = Object.keys(person).map(function(k) { return person[k] }).join(',');
// > person = '1,James,None'
Both options will give you a string that you could search for just the values you're interested in. In the latter scenario, you'd need to add some formatting to turn the outcome into exactly what you have requested, but then given the question I'm assuming presentation is not a big deal.
However, if at all possible, I think your code would much more closely match your intentions if you instead modified the search algorithm to inspect values and not entire objects. You haven't shown us any of the code doing the searching, though, so I can't really add suggestions for that at this point.

How to access data in an array of objects in another file

Basically, I'm working on an HTML page where I have to print out text that's stored in another .js file. The thing is, the text is stored in an object... in an array... in a function... in a variable. So there's lots of digging to be done, just to access ONE piece of data.
Here's the data (or something similar) in the accompanying .js file:
var TestDataSet= (function() {
var reviews = [
{ Id: "abcd1234",
Title: "This Is Title Text",
Number: 5,
Body: "text",
CreateDate: new Date(2012,5,23,14,12,10,0),
Owner: {
Id: "Person1234",
Name: "James Smith",
}
},
]
How would I make the browser return "abcd1234" from the First ID? How about "Person1234" from the nested ID?
The best I've got so far is this:
var data1 = new reviews;
console.log(data1.reviews[0].Id);
But this does nothing. I get a whole lot of "not defined" errors.

Apparently you haven't posted your complete function.
However, I suggest to return the array value as in a getter function and use it if is safe for the rest of your script:
var TestDataSet= (function() {
var reviews = [
{ Id: "abcd1234",
Title: "This Is Title Text",
Number: 5, Body: "text",
CreateDate: new Date(2012,5,23,14,12,10,0),
Owner: {
Id: "Person1234",
Name: "James Smith",
}
},];
return reviews;
});
Then you can access data using:
TestDataSet[0].Id

MongoDB mapReduce method unexpected results

I have 100 documents in my mongoDB, assuming each of them are possible duplicate with other document(s) in different conditions, such as firstName & lastName, email and mobile phone.
I am trying to mapReduce these 100 documents to have the key-value pairs, like grouping.
Everything works fine until I have the 101st duplicate records in the DB.
The output of the mapReduce result for the other documents which are duplicate with the 101st records are corrupted.
For example:
I am working on firstName & lastName now.
When the DB contains 100 documents, I can have the result containing
{
_id: {
firstName: "foo",
lastName: "bar,
},
value: {
count: 20
duplicate: [{
id: ObjectId("/*an object id*/"),
fullName: "foo bar",
DOB: ISODate("2000-01-01T00:00:00.000Z")
},{
id: ObjectId("/*another object id*/"),
fullName: "foo bar",
DOB: ISODate("2000-01-02T00:00:00.000Z")
},...]
},
}
It is what exactly I want, but...
when the DB contains more than 100 possible duplicated documents, the result became like this,
Let's say the 101st documents is
{
firstName: "foo",
lastName: "bar",
email: "foo#bar.com",
mobile: "019894793"
}
containing 101 documents:
{
_id: {
firstName: "foo",
lastName: "bar,
},
value: {
count: 21
duplicate: [{
id: undefined,
fullName: undefined,
DOB: undefined
},{
id: ObjectId("/*another object id*/"),
fullName: "foo bar",
DOB: ISODate("2000-01-02T00:00:00.000Z")
}]
},
}
containing 102 documents:
{
_id: {
firstName: "foo",
lastName: "bar,
},
value: {
count: 22
duplicate: [{
id: undefined,
fullName: undefined,
DOB: undefined
},{
id: undefined,
fullName: undefined,
DOB: undefined
}]
},
}
I found another topic on stackoverflow having the similar issue like me, but the answer does not work for me
MapReduce results seem limited to 100?
Any ideas?
Edit:
Original source code:
var map = function () {
var value = {
count: 1,
userId: this._id
};
emit({lastName: this.lastName, firstName: this.firstName}, value);
};
var reduce = function (key, values) {
var reducedObj = {
count: 0,
userIds: []
};
values.forEach(function (value) {
reducedObj.count += value.count;
reducedObj.userIds.push(value.userId);
});
return reducedObj;
};
Source code now:
var map = function () {
var value = {
count: 1,
users: [this]
};
emit({lastName: this.lastName, firstName: this.firstName}, value);
};
var reduce = function (key, values) {
var reducedObj = {
count: 0,
users: []
};
values.forEach(function (value) {
reducedObj.count += value.count;
reducedObj.users = reducedObj.users.concat(values.users); // or using the forEach method
// value.users.forEach(function (user) {
// reducedObj.users.push(user);
// });
});
return reducedObj;
};
I don't understand why it would fail as I was also pushing a value (userId) to reducedObj.userIds.
Are there some problems about the value that I emitted in map function?

Explaining the problem
This is a common mapReduce trap, but clearly part of the problem you have here is that the questions you are finding don't have answers that explain this clearly or even properly. So an answer is justified here.
The point in the documentation that is often missed or at least misunderstood is here in the documentation:
MongoDB can invoke the reduce function more than once for the same key. In this case, the previous output from the reduce function for that key will become one of the input values to the next reduce function invocation for that key.
And adding to that just a little later down the page:
the type of the return object must be identical to the type of the value emitted by the map function.
What this means in the context of your question is that at a certain point there are "too many" duplicate key values being passed in for a reduce stage to act on this in one single pass as it will be able to do for a lower number of documents. By design the reduce method is called multiple times, often taking the "output" from data that is already reduced as part of it's "input" for yet another pass.
This is how mapReduce is designed to handle very large datasets, by processing everything in "chunks" until it finally "reduces" down to a singular grouped result per key. This is why the next statement is important is that what comes out of both emit and the reduce output needs to be structured exactly the same in order for the reduce code to handle it correctly.
Solving the problem
You correct this by fixing up how you are both emitting the data in the map and how you also return and process in the reduce function:
db.collection.mapReduce(
function() {
emit(
{ "firstName": this.firstName, "lastName": this.lastName },
{ "count": 1, "duplicate": [this] } // Note [this]
)
},
function(key,values) {
var reduced = { "count": 0, "duplicate": [] };
values.forEach(function(value) {
reduced.count += value.count;
value.duplicate.forEach(function(duplicate) {
reduced.duplicate.push(duplicate);
});
});
return reduced;
},
{
"out": { "inline": 1 },
}
)
The key points can be seen in both the content to emit and the first line of the reduce function. Essentially these present a structure that is the same. In the case of the emit it does not matter that the array being produced only has a singular element, but you send it that way anyhow. Side by side:
{ "count": 1, "duplicate": [this] } // Note [this]
// Same as
var reduced = { "count": 0, "duplicate": [] };
That also means that the remainder of the reduce function will always assume that the "duplicate" content is in fact an array, because that is how it came as original input and is also how it will be returned:
values.forEach(function(value) {
reduced.count += value.count;
value.duplicate.forEach(function(duplicate) {
reduced.duplicate.push(duplicate);
});
});
return reduced;
Alternate Solution
The other reason for an answer is that considering the output you are expecting, this would in fact be much better suited to the aggregation framework. It's going to do this a lot faster than mapReduce can, and is even far more simple to code up:
db.collection.aggregate([
{ "$group": {
"_id": { "firstName": "$firstName", "lastName": "$lastName" },
"duplicate": { "$push": "$$ROOT" },
"count": { "$sum": 1 }
}},
{ "$match": { "count": { "$gt": 1 } }}
])
That's all it is. You can write out to a collection by adding an $out stage to this where required. But basically either mapReduce or aggregate, you are still placing the same 16MB restriction on the document size by adding your "duplicate" items into an array.
Also note that you can simply do something that mapReduce cannot here, and just "omit" any items that are not in fact a "duplicate" from the results. The mapReduce method cannot do this without first producing output to a collection and then "filtering" the results in a separate query.
That core documentation itself quotes:
NOTE
For most aggregation operations, the Aggregation Pipeline provides better performance and more coherent interface. However, map-reduce operations provide some flexibility that is not presently available in the aggregation pipeline.
So it's really a case of weighing up which is better suited to the problem at hand.