javascript nested forEach, typeError

Question

I am doing double forEach, but getting a typeError, while trying to reconstruct another object:

const users = [
    {teacher: [
        {file: 'chemistry', size: '2MB'},
        {file: 'math', size: '1MB'}]
    }, {student: [
        {file: 'chemistry', size: '3MB'},
        {file: 'math', size: '4MB'}]
    }
];

let final = {};

users.forEach(function(i) {

    i.forEach(function(j){
        let filesizestring = 'newfilesize'+j.size;
        final[j] = j;
        final.j[j.file] = filesizestring;
    })
})

and the expected result is:

{teacher: {
        chemistry: 'newfilesize2MB',
        math: 'newfilesize1MB'
    }, 
student: {
        chemistry: 'newfilesize3MB',
        math: 'newfilesize4MB'
    }
}

could somebody help me fix this?

update

if nested forEach is not possible, how can i achieve the same result?

Cause you dont have a nested array, so you cant use a nested forEach loop. You should definetly change your datastructure, it makes no sense at all. What do you want to do? — Jonas Wilms
– Jonas Wilms, Commented Jan 22, 2018 at 14:53

Ele · Accepted Answer · 2018-01-22 16:59:32Z

4

You're executing a loop over i -> i.forEach(function(j), however, i is an object, so you need to loop over the objects key-value pairs using for (key in i) and for each key get the array as follow i[key]

const users = [{
  teacher: [{
      file: 'chemistry',
      size: '2MB'
    },
    {
      file: 'math',
      size: '1MB'
    }
  ]
}, {
  student: [{
      file: 'chemistry',
      size: '3MB'
    },
    {
      file: 'math',
      size: '4MB'
    }
  ]
}];

let final = {};

users.forEach(function(i) {
  for (key in i) {
    var obj = {};
    i[key].forEach(function(j) {
      let filesizestring = 'newfilesize' + j.size;
      obj[j.file] = filesizestring;
    });
    final[key] = obj;
  }
});

console.log(final);

Hope it helps!

edited Jan 22, 2018 at 16:59

answered Jan 22, 2018 at 14:57

Ele

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6 Comments

moaningalways Over a year ago

can you give an answer with map method?

Adrien Brunelat Over a year ago

@moaningalways Why do I have a strange feeling like you are not the one really asking for an answer with a map() method. Is there a reason why you would want a map method?

Ele Over a year ago

@JonasW. Why a downvote? Just for the missing explanation? At least can you comment and wait for my update.

Ele Over a year ago

Whatever, let's close the conversation!

Ele Over a year ago

@JonasW. you're right with that fact, but you can ask for an explanation before hitting a downvote. I do that, have a good day.

|

GrumpyCrouton · Accepted Answer · 2018-01-22 15:52:09Z

3

You can do it using reduce like this:

const users = [{
    teacher: [{
            file: 'chemistry',
            size: '2MB'
        },
        {
            file: 'math',
            size: '1MB'
        }
    ]
}, {
    student: [{
            file: 'chemistry',
            size: '3MB'
        },
        {
            file: 'math',
            size: '4MB'
        }
    ]
}];

let final = users.reduce((a, u) => (k = Object.keys(u)[0], { ...a,
    [k]: u[k].reduce((a, s) => ({ ...a,
        [s.file]: 'newfilesize' + s.size
    }), {})
}), {})

console.log(final)

edited Jan 22, 2018 at 15:52

GrumpyCrouton

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answered Jan 22, 2018 at 15:11

mdatsev

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1 Comment

Ele Over a year ago

This is a nice way!

Ali Faris · Accepted Answer · 2018-01-22 15:27:22Z

1

i is object not array in forEach so you cannot call forEach on object , what you can do is get the array from the object and apply forEach on that array

    const users = [
        {teacher: [{ file: 'chemistry', size: '2MB' },{ file: 'math', size: '1MB' }]}, 
        {student: [{ file: 'chemistry', size: '3MB' },{ file: 'math', size: '4MB' }]}
    ];

    let final = {};

    users.forEach(function (user) {
        let key = Object.keys(user)[0];
        let obj = {};
        user[key].forEach(function(file){
            obj[file.file] = 'newfilesize' + file.size;
        });

        final[key] = obj;
    });

    console.log(final);

edited Jan 22, 2018 at 15:27

answered Jan 22, 2018 at 15:01

Ali Faris

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1 Comment

moaningalways Over a year ago

the output is not exactly the expected one. could you please modify it? fix it?

tevemadar · Accepted Answer · 2018-01-22 15:36:02Z

I just do not like not-having a function, even if it has a silly name.
Otherwise the issue is what appears in other answers too: your top-level variable users is an array, and has forEach. But this array contains objects, which have no forEach, but you can use for in instead (that is supposed to be a link) to get teacher/student. And the last level is again an array, which you can address with forEach.
So there is a nested forEach after all, just it is nested into a for in.

const users = [
    {teacher: [
        {file: 'chemistry', size: '2MB'},
        {file: 'math', size: '1MB'}]
    },
    {student: [
        {file: 'chemistry', size: '3MB'},
        {file: 'math', size: '4MB'}]
    }
];

function magic(coll){
  var res={};
  coll.forEach(function(user){
    for(misc in user){
      user[misc].forEach(function(subject){
        this[subject.file]="newfilesize"+subject.size;
      },res[misc]={});
    }
  });
  return res;
}

console.log(magic(users));

(If someone has a good word for misc, please suggest)

Adrien Brunelat · Accepted Answer · 2018-01-22 14:58:38Z

0

Did you try with a data structure like this instead?

const users = [
    {type: "teacher",
     files: [
        {file: 'chemistry', size: '2MB'},
        {file: 'math', size: '1MB'}]
    }, {type: "student",
        files: [
        {file: 'chemistry', size: '3MB'},
        {file: 'math', size: '4MB'}]
    }
];

let final = {};

users.forEach(function(i) {

    i.files.forEach(function(j){
        let filesizestring = 'newfilesize'+j.size;
        final[j] = j;
        final[j][j.file] = filesizestring;  // <-- What?
    })
})

You'll have to explain the line on which I've put a comment though, I don't get it.

answered Jan 22, 2018 at 14:58

Adrien Brunelat

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Comments

I wrestled a bear once. · Accepted Answer · 2018-01-22 15:10:03Z

It's not safe to turn an array into an object like that unless you're sure there is only one item in the array. This answer assumes there may be multiple, and so it returns an array containing the structure you want. If you're sure there will only be one you can just use final[0] as the result array would, in that case, only contain a a single element as well.

const users = [{
  teacher: [{
    file: 'chemistry',
    size: '2MB'
  }, {
    file: 'math',
    size: '1MB'
  }]
}, {
  student: [{
    file: 'chemistry',
    size: '3MB'
  }, {
    file: 'math',
    size: '4MB'
  }]
}];

final = [];
users.forEach(i => {
  let obj = {};
  let name = i.teacher ? "teacher" : "student";
  obj[name] = {};
  i[name].forEach(j => obj[name][j.file] = 'newfilesize' + j.size);
  final.push(obj);
});

console.log(final);

Jonas Wilms · Accepted Answer · 2018-01-22 15:12:57Z

0

I would change the datastructure to this (your current one makes no sense):

  const users = [
   {
     type:"teacher",
     files: [
       {file: 'chemistry', size: '2MB'},
       {file: 'math', size: '1MB'}
     ]
   }, {
     type:"student",
     files: [
       {file: 'chemistry', size: '3MB'},
       {file: 'math', size: '4MB'}
     ]
   }
 ];

Now you can iterate over the users and its files:

for(const user of users){
  for(const file of user.files){
    file.size = "newfilesize" + file.size;
  }
 }

edited Jan 22, 2018 at 15:12

answered Jan 22, 2018 at 14:59

Jonas Wilms

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2 Comments

moaningalways Over a year ago

ok, but your answer is still not giving the right output.. could you please fix it?

Jonas Wilms Over a year ago

@moaningalways no i could not. Your expected output makes no sense and is not useful at all.

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