Getting an object out of an array in javascript

Question

I have an array of objects (student objects which have properties like studentID, grade, dob, registeredDate) and I have another array of objects (books objects which have properties studentID, bookID, bookISBN).

It's a Web app for managing the library at a small school. What I want to do is that when a book (say has title Welcome Home) has been borrowed by student with studentID 4, when you try to lend that book (of course a copy since the library can have a number of them in stock) to someone else, the student with studentID 4 shouldn't be available in the list of students who are eligible to get that book (unless if the student had returned the book first).

The booksList Array is like below:

[
{
bookEdition: "2nd edition",
bookISBN: 9876533278765,
bookStatus: 0,
bookTitle: "Real Machines",
dateTaken: "2018-10-28",
returnDate: "2018-11-27",
studentID: "0000003"
},

{
bookEdition: "2015 edition",
bookISBN: 9876532226712,
bookStatus: 0,
bookTitle: "Real Machines",
dateTaken: "2018-08-28",
returnDate: "2018-09-27",
studentID: "0000004"
}
];

The studentsList is as below:

[
{
bio: "Needs extra work. Has problems with mathematics",
birthday: "2005-05-12",
className: "grade 5",
fullname: "Bridget Friday",
gender: "Female",
parentName: "Josam Friday",
studentID: "0000003",
studentStatus: "approved"
}, 

{
bio: "A bit naughty but intelligent. Pay close attention to his diet.",
birthday: "2003-11-07",
className: "grade 6",
fullname: "Charles Ben",
gender: "Male",
parentName: "Chris Ben",
studentID: "0000004",
studentStatus: "approved"
}
];

Now, I was trying to use the filter function but it doesn't give me the results I want. The thing that links the two array objects and the objects in them is the studentID.

I tried

var legitStudents = studentsList.filter(el => {
  return !booksList.includes(el.studentID);
});

The above doesn't work. The arrays (studentList and booksList) are fetched dynamically and I can't tell which studentIDs are in booksList.

How can I get this to work as I want them?

We can't help without knowing what the format of studentsList and booksList is. I'd tend to assume booksList isn't an array of student IDs, which is what your code expects it to be. Please update your question with a minimal reproducible example demonstrating the problem, ideally a runnable one using Stack Snippets (the [<>] toolbar button; here's how to do one). — T.J. Crowder
– T.J. Crowder, Commented Oct 28, 2018 at 11:19
!booksList.includes(el.studentID); this isn't right since booksList is not an array of studentIDs. It's an array objects — Sajal Preet Singh
– Sajal Preet Singh, Commented Oct 28, 2018 at 11:20
@T.J.Crowder Right now I can't put it here, I'm using Angular 6 and it's just a bit complex for me right now. I'll just log the studentList array and the booksList array so you can see what they contain. — Kingsley
– Kingsley, Commented Oct 28, 2018 at 11:23
@SajalPreetSingh I understand that. So what could be the best way to go about it? — Kingsley
– Kingsley, Commented Oct 28, 2018 at 11:24
@Kingsley - It's better to ask a properly-formed question. If you can't do that right now, I suggest deleting the question and posting it when you can. — T.J. Crowder
– T.J. Crowder, Commented Oct 28, 2018 at 11:29

Rocky Sims · Accepted Answer · 2018-10-28 11:27:32Z

1

return !booksList.includes(el.studentID);

should be

return !booksList.map(i => i.studentID).includes(el.studentID);

As a couple people said in the comments of your question, the problem is your code is expecting booksList to be an array of studentIDs. Since it's actually a list of books that have been checked out by students, you first need to make an array of all studentIDs in booksList then you can use includes on the resulting array. See map.

edited Oct 28, 2018 at 11:27

answered Oct 28, 2018 at 11:22

Rocky Sims

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1 Comment

Kingsley Over a year ago

This also works and it's pretty clear to understand. Big thanks

Sajal Preet Singh · Accepted Answer · 2018-10-28 11:29:47Z

1

You can use Rocky Sims' solution or you can try this as well

var legitStudents = studentList.filter(student => {
  return !booksList.filter(book => book.studentID === student.studentID).length;
});

answered Oct 28, 2018 at 11:29

Sajal Preet Singh

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2 Comments

Kingsley Over a year ago

Wow! Works like a charm. I just don't understand why there should be length at the end. If I take it out it doesn't work. Big thanks, though. It just works out fine...

Sajal Preet Singh Over a year ago

booksList.filter(book => book.studentID === student.studentID) gives out an array of either length 1 or 0. So ! and .length to get the necessary boolean

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