I have the following array of objects:

[
   {
     message: 'This is a test',
     from_user_id: 123,
     to_user_id: 567
   },
   {
     message: 'Another test.',
     from_user_id: 123,
     to_user_id: 567
   },
   {
     message: 'A third test.',
     from_user_id: '456',
     to_user_id: 567
   }
]

How do I construct a new array of objects where the outermost object key is based on a mon key found in the original array?

This is what I'm after:

[
  {
    123: [
      {
        message: 'This is a test',
        from_user_id: 123,
        to_user_id: 567
      },
      {
        message: 'Another test.',
        from_user_id: 123,
        to_user_id: 567
      }
    ]
  },
  {
    456: [
      {
        message: 'A third test.',
        from_user_id: '456',
        to_user_id: 567
      }
    ]
  }
]

Notice how in the first array, the user ID of 123 shows up in two objects. That would be the object key for the first element in the new array.

I have the following array of objects:

[
   {
     message: 'This is a test',
     from_user_id: 123,
     to_user_id: 567
   },
   {
     message: 'Another test.',
     from_user_id: 123,
     to_user_id: 567
   },
   {
     message: 'A third test.',
     from_user_id: '456',
     to_user_id: 567
   }
]

How do I construct a new array of objects where the outermost object key is based on a mon key found in the original array?

This is what I'm after:

[
  {
    123: [
      {
        message: 'This is a test',
        from_user_id: 123,
        to_user_id: 567
      },
      {
        message: 'Another test.',
        from_user_id: 123,
        to_user_id: 567
      }
    ]
  },
  {
    456: [
      {
        message: 'A third test.',
        from_user_id: '456',
        to_user_id: 567
      }
    ]
  }
]

Notice how in the first array, the user ID of 123 shows up in two objects. That would be the object key for the first element in the new array.

• javascript
• arrays
• json
• multidimensional-array
• javascript-objects

4 Answers 4

You could use an object and take the from_user_id property as key for the object. Then push the actual object to the group. For getting the final result, iterate the keys of groups and build a new object for any group.

var data = [{ message: 'This is a test', from_user_id: 123, to_user_id: 567 }, { message: 'Another test.', from_user_id: 123, to_user_id: 567 }, { message: 'A third test.', from_user_id: '456', to_user_id: 567 }],
    groups = Object.create(null),
    result;

data.forEach(function (a) {
    groups[a.from_user_id] = groups[a.from_user_id] || [];
    groups[a.from_user_id].push(a);    
});

result = Object.keys(groups).map(function (k) {
    var temp = {};
    temp[k] = groups[k];
    return temp;
});

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Together with a single loop approach

var data = [{ message: 'This is a test', from_user_id: 123, to_user_id: 567 }, { message: 'Another test.', from_user_id: 123, to_user_id: 567 }, { message: 'A third test.', from_user_id: '456', to_user_id: 567 }],
    result = data.reduce(function (groups) {
        return function (r, a) {
            var temp = {};
            if (!groups[a.from_user_id]) {
                groups[a.from_user_id] = [];
                temp[a.from_user_id] = groups[a.from_user_id]; 
                r.push(temp);
            }
            groups[a.from_user_id].push(a);
            return r;
        };
    }(Object.create(null)), []);

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

You could go full functional get all the key filter it out and map it back as a json object

var b = a.map(key => key['from_user_id'])
var c = {}
b.map(elt => c[elt] = a.filter(k => k.from_user_id == elt))
console.log(c)

You just need to initialize a results array, loop over your data array and check if the iterated from_user_id exists in the results array, push the iterated object on it, otherwise create a new object with the new from_user_id key.

This is how should be your code:

var results = [];
arr.forEach(function(obj){
    let id = obj["from_user_id"];
    if(!results.some(function(r){
       return r[id];
    })){
        let el = {}; 
        el[id] = [];
        el[id].push(obj);
        results.push(el);
    }else{
        results.forEach(function(res){
            if(res[id]){
              res[id].push(obj);
            }
        });
    }
});

Demo:

var arr = [{
  message: 'This is a test',
  from_user_id: 123,
  to_user_id: 567
}, {
  message: 'Another test.',
  from_user_id: 123,
  to_user_id: 567
}, {
  message: 'A third test.',
  from_user_id: 456,
  to_user_id: 567
}];


var results = [];
arr.forEach(function(obj){
    let id = obj["from_user_id"];
    if(!results.some(function(r){
       return r[id];
    })){
        let el = {}; 
        el[id] = [];
        el[id].push(obj);
        results.push(el);
    }else{
        results.forEach(function(res){
            if(res[id]){
              res[id].push(obj);
            }
        });
    }
});
console.log(results);

var users = [
             {
               message: 'This is a test',
               from_user_id: 123,
               to_user_id: 567
             },
             {
               message: 'Another test.',
               from_user_id: 123,
               to_user_id: 567
             },
             {
               message: 'A third test.',
               from_user_id: '456',
               to_user_id: 567
             }
          ];
console.log(_.groupBy(users,'from_user_id'))

<script src="https://cdnjs.cloudflare./ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>

Using Lodash will make it super easy for you. Assuming you have :

var users = [
             {
               message: 'This is a test',
               from_user_id: 123,
               to_user_id: 567
             },
             {
               message: 'Another test.',
               from_user_id: 123,
               to_user_id: 567
             },
             {
               message: 'A third test.',
               from_user_id: '456',
               to_user_id: 567
             }
          ];

just in one line with Lodash and you'll get want you want exactly:

users = _.groupBy(users,'from_user_id')