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How to use filter(), map() to work with values fo an array? JavaScript

My task is:
Implement the function duplicateStudents(), which gets the variable
"students" and filters for students with the same matriculation
number. Firstly, project all elements in students by matriculation
number. After that you can filter for duplicates relatively easily. At
the end project using the following format: { matrikelnummer:
(matrikelnummer), students: [ (students[i], students[j], ... ) ] }.
Implement the invalidGrades() function, which gets the variable "grades"
and filters for possibly incorrect notes. For example, in order to
keep a manual check as low as possible, the function should determine
for which matriculation numbers several grades were transmitted for
the same course. Example: For matriculation number X, a 2. 7 and a 2.
3 were transmitted for course Y. However, the function would also take
into account the valid case, i. e. for matriculation number X, once a
5,0 and once a 2,3 were transmitted for course Y.
In this task you should only use map(), reduce(), and filter(). Do not
implement for-loops.
function duplicateStudents(students) {
return students
// TODO: implement me
}
function invalidGrades(grades) {
return grades
.map((s) => {
// TODO: implement me
return {
matrikelnummer: -1/* put something here */,
grades: []/* put something here */,
};
})
.filter((e) => e.grades.length > 0)
}
The variables students and grades I have in a separate file. I know it might be helpful to upload the files too, but one is 1000 lines long, the other 500. That’s why I’m not uploading them. But I hope it is possible to do the task without the values. It is important to say that the values are represented as an array

I'll give you an example of using reduce on duplicateStudents, that's not returning the expected format but you could go from there.
const duplicateStudents = (students) => {
const grouping = students.reduce((previous, current) => {
if (previous[current.matrikelnummer]) previous[current.matrikelnummer].push(current); // add student if matrikelnummer already exist
else previous[current.matrikelnummer] = [current];
return previous;
}, {});
console.log(grouping);
return //you could process `grouping` to the expected format in here
};
here's preferences for you:
map
filter
reduce

Apply variable number of filter conditions to a javascript array while traversing it only once?

I have a javascript array of nested data that holds data which will be displayed to the user.
The user would like to be able to apply 0 to n filter conditions to the data they are looking at.
In order to meet this goal, I need to first find elements that match the 0 to n filter conditions, then perform some data manipulation on those entries. An obvious way of solving this is to have several filter statements back to back (with a conditional check inside them to see if the filter needs to be applied) and then a map function at the end like this:
var firstFilterList = _.filter(myData, firstFilterFunction);
var secondFilterList = _.filter(firstFilterList, secondFilterFunction);
var thirdFilterList = _.filter(secondFilterList, thirdFilterFunction);
var finalList = _.map(thirdFilterList, postFilterFunction);
In this case however, the javascript array would be traversed 4 times. A way to get around this would be to have a single filter that checks all 3 (or 0 to n) conditions before determining if there is a match, and then, inside the filter at the end of the function, doing the data manipulation, however this seems a bit hacky and makes the "filter" responsible for more than one thing, which is not ideal. The upside would be that the javascript Array is traversed only once.
Is there a "best practices" way of doing what I am trying to accomplish?
EDIT: I am also interested in hearing if it is considered bad practice to perform data manipulation (adding fields to javascript objects etc...) within a filter function.

You could collect all filter functions in an array and check every filter with the actual data set and filter by the result. Then take your mapping function to get the wanted result.
var data = [ /* ... */ ],
filterFn1 = () => Math.round(Math.random()),
filterFn2 = (age) => age > 35,
filterFn3 = year => year === 1955,
fns = [filterFn1, filterFn2, filterFn2],
whatever = ... // final function for mapping
result = data
.filter(x => fns.every(f => f(x)))
.map(whatever);

One thing you can do is to combine all those filter functions into one single function, with reduce, then call filter with the combined function.
var combined = [firstFilterFunction, seconfFilterFunction, ...]
.reduce((x, y) => (z => x(z) && y(z)));
var filtered = myData.filter(combined);

resolve field function before sort

I am creating a graphql server using express, and I have a resolver that can transform my fields as per input from the user query.
The transformer that I am using is returning a function, which is the cause of my issues.
I want to sort my result by some user determined field, but since the field is a function, it won't work.
So the resolver looks like this:
const resolver = (req, param) => {
return {
history: async input => {
let size = input.pageSize || 3;
let start = (input.page || 0) * size;
let end = start + size;
let sortField = (input.sort || {}).field || 'timestamp';
return fs.promises.readFile("./history/blitz.json", "utf8").then(data =>
JSON.parse(data)
.slice(start, end)
.map(job => historyTransformer(job))
.sort((a,b) => a[sortField] > b[sortField] ? 1 : a[sortField] < b[sortField] ? -1 : 0)
);
}
};
};
and the transformer:
const historyTransformer = job => {
return {
...job,
timestamp: input =>
dateFormat(job.timestamp, input.format || "mm:hh dd-mm-yyyy")
};
};
I am not sure if I am missing something but is there an easy way of resolving the function call before starting the sorting?

GraphQL fields are resolved in a hierarchal manner, such that the history field has to resolve before any of its child fields (like timestamp) can be resolved. If the child field's resolver transforms the underlying property and your intent is to somehow use that value in the parent resolver (in this case, to do some sorting), that's tricky because you're working against the execution flow.
Because you're working with dates, you should consider whether the format of the field even matters. As a user, if I sort by timestamp, I expect the results to be sorted chronologically. Even if the response is formatted to put the time first, I probably don't want dates with the same times but different years grouped together. Of course, I don't know your business requirements and it still doesn't solve the problem if we're working with something else, like translations, which would cause the same problem.
There's two solutions I can think of:
Update your schema and lift the format argument into the parent field. This is easier to implement, but obviously not as nice as putting the argument on the field it applies to.
Keep the argument where it is, but parse the info parameter passed to the resolver to determine the value of the argument inside the parent resolver. This way, you can keep the argument on the child field, but move the actual formatting logic into the parent resolver.

How does manipulating multiple checkbox selection code work?

I am following ag-grid's official tutorial:
https://www.ag-grid.com/angular-getting-started/?utm_source=ag-grid-readme&utm_medium=repository&utm_campaign=github
I reached the part where I have to manipulate the information regarding the selected checkboxes. However, the documentation is not detailed; It does not explain how the code actually works. Perhaps, this makes sense since it is not their job to explain in detail. However, for someone like me who doesn't have solid experience with working with angular technology and who wants to truly understand how things work, this is troublesome!
In the html file, I have been asked to add this:
<button (click)="getSelectedRows()">Get Selected Rows</button>
And in app.component.ts, I have been asked to add this:
getSelectedRows() {
const selectedNodes = this.agGrid.api.getSelectedNodes();
const selectedData = selectedNodes.map(node => node.data);
const selectedDataStringPresentation = selectedData.map( node => node.make + ' ' + node.model).join(', ');
alert('Selected nodes: ${selectedDataStringPresentation}');
}
If someone could explain what the typescript code is doing exactly, that would be very generous.
Thank you!

I guess agGrid is the service storing your mock values, this simply gets an array of data from somwhere.
selectedData is another array that is created by transforming (transforms the array while providing a new reference, thus not modifying the original array) the selectedNodes array and only selecting the data property of each node.
selectedDataStringPresentation is the same, but this time it provides an array of formatted strings by merging the properties make and model of each object from selectedData.
What you probably fail to grasp is the usage of the ES6 (JavaScript standard) functions that are used here, and especially the map() function.
Here is the official documentation of the map() function for arrays : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map
Simply explained, it is a function that iterates over an array, and transforming each object by applying the function declared in the map, returning the results in a new array.
If you take the example of selectedData, you can translate the operation into this :
Loop over each object of selectedNodes, and return only the property data of the current object. Push the results in a new array.
This is especially useful when you want to work on different arrays that serve different purposes. For example, imagine you have a service that contains a list of objects. A backend service will provide you an array of numbers representing the IDs of the objects you have in your service.
You can then use the map() function to transform the array of IDs into an array of your Objects stored in your service easily.

Darn #Alex Beugnet(upvote) beat me to the punch! I'm going to post anyway as I was in the middle of writing it all.
Firstly I'm not sure how much of TypeScript you already know, I apologize if much of these becomes trivial, the purpose is only to ensure maximum clarification to the question in understanding the logic.
In the Enable Selection portion of the guide, you are essentially enabling multiple row selection in the grid and having the button return the data from those selected rows.
In order to see what's happening with the getMultipleRows() function, it would be best to visualize it via the Debugger provided in browsers, I'm using Chrome Developer Tools (hit F12), I would highly recommend it for understanding what is happening in the logic.
const selectedNodes
Let's start by selecting say 2 rows, I have selected the Porsche Boxster 72000 and Ford Mondeo 32000. After selecting them I click on the 'Get Selected Rows' button which triggers the getSelectedRows() function:
const selectedNodes = this.agGrid.api.getSelectedNodes();
The above line is assigning the constant variable 'selectedNodes' the RowNodes from AgGrid. Here you are using the AgGridNg2 method getSelectedNodes() to return the selected node data, which you would be returned an array in the form of:
[RowNode, RowNode] //(each for the row we have selected)
Looking into a RowNode we get:
These are all the RowNode properties provided by the AgGrid framework, you can ignore all of these object properties as you are only concerned with the 'data' property as you'll see in the next line of code!
const SelectedData
const selectedData = selectedNodes.map(node => node.data);
Here we are setting 'selectedData' as an array of RowNode.data, basically trying to get the data property from the RowNodes into an array.
The above line can basically be assumed as:
let selectedData = [];
for (let i = 0; i <= selectedNodes.length - 1; i++){
selectedData[i] = selectedNodes[i].data;
}
In which we are just trying to get the data property into a new constant variable 'selectedData'. Look at the documentation in order to better understand this: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map
const selectedData would be returned as:
[
{
make: "Porsche",
model: "Boxster",
price: 72000,
},
{
make: "Ford",
model: "Mondeo",
price: 32000
}
]
const selectedDataStringPresentation
const selectedDataStringPresentation = selectedData.map( node => node.make + ' ' + node.model).join(', ');
We take the selectedData array and concatenate the Make and Model as a single element for the array and adding a comma in the end. We would get "Porsche Boxter, Ford Mondeo".
let selectedData = [
{
make: "Porsche",
model: "Boxster",
price: 72000,
},
{
make: "Ford",
model: "Mondeo",
price: 32000
}
]
let selectedDataStringPresentation = [];
for (let i = 0; i <= selectedData.length - 1; i++){
selectedDataStringPresentation[i] = selectedData[i].make + ' ' + selectedData[i].model;
}
selectedDataStringPresentation = selectedDataStringPresentation.join(', ');
console.log(selectedDataStringPresentation);
alert()
And the last line,
alert('Selected nodes: ${selectedDataStringPresentation}');
You are going to send an alert in the browser that will display the selectedDataStringPresentation array.

MongoDB count and results

I would really appreciate if someone could help me with something: I need to make a normal query to the database but, as my collection is very large (10000 documents) I need to do the query and use $limit and $skip. That part I solved but now I want to have a count to all the documents, even if the returned ones are less. The output should be something like this:
{
count: 1150,
data: [/*length of 50*/]
}
Could anyone help please? Thank you very much.

Since you mentioned you are making a normal query, its not wise to go for aggregation. find() will be a much better option here. Instead you can use the find query itself. The commands to do this in mongoDB console is shown below:
> var docs = db.collection.find().skip(10).limit(50)
> docs.count()
189000
> docs.length()
50

You can do this using one query itself. In Node.js and Express.js, you will have to use it like this to be able to use the "count" function along with the toArray's "result".
var curFind = db.collection('tasks').find({query});
Then you can run two functions after it like this (one nested in the other)
curFind.count(function (e, count) {
// Use count here
curFind.skip(0).limit(10).toArray(function(err, result) {
// Use result here and count here
});
});

I don't think it is possible in a single query to get the total count of the result along with the paginated data without using aggregation.
You can probably achieve this via aggregation but since you mentioned, your collection is very large, you should avoid it and break the query into two parts. I'm providing you an example of considering user collection having a rating field with more than 10,000 records:
var finalResult = {};
var query = {
rating: {$gt: 2}
};
// Get first 50 records of users having rating greater than 2
var users = db.user.find(query).limit(50).skip(0).toArray();
// Get total count of users having rating greater than 2
var totalUsers = db.user.cound(query);
finalResult.count = totalUsers;
finalResult.data = users;
And your final output can be like:
finalResult == {
count: 1150,
data: [/*length of 50 users*/]
}
Hope, this make sense to you. Some of the famous technologies like Grails internally do that to achieve pagination.
Another cleaner approach could be:
var finalResult = {};
var query = {
rating: {$gt: 2}
};
var cursor = db.user.find(query).limit(50).skip(0);
// Get total count of users having rating greater than 2
// By default, the count() method ignores the effects of the cursor.skip() and cursor.limit()
finalResult.count = cursor.count();;
finalResult.data = cursor.toArray();

As mentioned by Sarath Nair you can use count, as it ignores skip and limit. Check: MongoDB Count
By the way, this is duplication from another StackOverflow question: Limiting results in MongoDB but still getting the full count?