Javascript get array of objects from array of objects in array of objects

Question

I have an array with registrations, inside this array is an array of students.
Now I want an array of all the students with only firstName, lastName and email.

registrations array

[
  0: {
    date: "2019-04-08T13:51:10.215Z"
    onlyVAT: false,
    students: [
      0: {
        email: "[email protected]",
        firstName: "Bennn",
        lastName: "test",
        phone: "0898989"
        ...
      }
    ]
  }
]

What I have so far:

 this.registrations.map(
    registration => registration.students.map(
        student => { return {
          firstName: student.firstName,
          lastName: student.lastName,
          email: student.email
          }}
      )
    );

but this returns an array of arrays

0: [
  0: {firstName: "Bennn", lastName: "test", email: "[email protected]"},
  1: ...
]

what I want is an array of (partial) student objects

[
  0: {firstName: "Bennn", lastName: "test", email: "[email protected]"},
  1: ...
]

ofcourse I could just loop and push to a new array but that's not what I want

Don't forget to assign the map result, it looks like it's not. — briosheje
– briosheje, Commented Jun 27, 2019 at 9:33
Should those be all student records from all registrations? Are there any duplicates? If yes, should they be removed? — Yevhen Horbunkov
– Yevhen Horbunkov, Commented Jun 27, 2019 at 9:38
@U25lYWt5IEJhc3RhcmQg It should be all students from all registrations, there are no duplicates, however, an examples that includes the removing of duplicates would be nice too! — Ruben
– Ruben, Commented Jun 27, 2019 at 9:40
also I don't know if this is the fastest solution, should I use filter instead? — Ruben
– Ruben, Commented Jun 27, 2019 at 9:54
@Ruben : if duplicates removal may be required, my code may do the job — Yevhen Horbunkov
– Yevhen Horbunkov, Commented Jun 27, 2019 at 10:48

jank · Accepted Answer · 2019-06-27 09:35:35Z

2

Use flat() or flatMap(). Example:

  const newArr = registrations.map(
    registration => registration.students.map(
      student => { return {
        firstName: student.firstName,
        lastName: student.lastName,
        email: student.email
      }}
    )
  ).flat();

  console.log(newArr);

answered Jun 27, 2019 at 9:35

jank

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Comments

Jack Bashford · Accepted Answer · 2019-06-27 09:37:12Z

1

Just use flatMap.

this.registrations.flatMap(...);

Or use reduce:

this.registrations.map(...).reduce((a, c) => [...a, c], []);

edited Jun 27, 2019 at 9:37

answered Jun 27, 2019 at 9:32

Jack Bashford

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1 Comment

Ziv Ben-Or Over a year ago

You can use [...a, c] instead of a.concat(c) 😎

Andriy · Accepted Answer · 2019-06-27 12:25:52Z

Here is a solution with two Array.reduce() functions and Object.values(), which will ensure that the output will contain only unique emails (in the example input I have two identical emails [email protected]):

const registrations = [
  {
    date: '2019-04-08T13:51:10.215Z',
    onlyVAT: false,
    students: [
      {
        email: '[email protected]',
        firstName: 'Bennn',
        lastName: 'test',
        phone: '0898989'
      },
      {
        email: '[email protected]',
        firstName: 'Bennn2',
        lastName: 'test2',
        phone: '0898989'
      }
    ]
  },
  {
    date: '2019-05-08T13:51:10.215Z',
    onlyVAT: false,
    students: [
      {
        email: '[email protected]',
        firstName: 'Bennn3',
        lastName: 'test3',
        phone: '0898989'
      },
      {
        email: '[email protected]',
        firstName: 'Bennn4',
        lastName: 'test4',
        phone: '0898989'
      },
      {
        email: '[email protected]',
        firstName: 'Bennn4',
        lastName: 'test4',
        phone: '0898989'
      }
    ]
  }
];


const result = registrations.reduce((res, {students}) => ({
  ...res,
  ...students.reduce((res2, {email, firstName, lastName}) => ({
    ...res2,
    [email]: {email, firstName, lastName}
  }), {})
}), {});

console.log(Object.values(result));

Nabin Paudyal · Accepted Answer · 2019-06-27 09:37:16Z

0

The result is obtained in such form since students is a field inside an object in the array.

You could simply apply a combination of concat and ... spread operator on the result obtained from map to get the final array of students.

const studentsMapped = this.registrations.map(
    registration => registration.students.map(
        student => { return {
            firstName: student.firstName,
            lastName: student.lastName,
            email: student.email
       }}
    )
);

const students = [].concat(...studentsMapped);

answered Jun 27, 2019 at 9:37

Nabin Paudyal

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