Group array items using object

First, in JavaScript it's generally not a good idea to iterate over arrays using for ... in. See Why is using "for...in" with array iteration a bad idea? for details.

So you might try something like this:

var groups = {};
for (var i = 0; i < myArray.length; i++) {
  var groupName = myArray[i].group;
  if (!groups[groupName]) {
    groups[groupName] = [];
  }
  groups[groupName].push(myArray[i].color);
}
myArray = [];
for (var groupName in groups) {
  myArray.push({group: groupName, color: groups[groupName]});
}

Using the intermediary groups object here helps speed things up because it allows you to avoid nesting loops to search through the arrays. Also, because groups is an object (rather than an array) iterating over it using for ... in is appropriate.

Addendum

FWIW, if you want to avoid duplicate color entries in the resulting arrays you could add an if statement above the line groups[groupName].push(myArray[i].color); to guard against duplicates. Using jQuery it would look like this;

if (!$.inArray(myArray[i].color, groups[groupName])) {
  groups[groupName].push(myArray[i].color);
}

Without jQuery you may want to add a function that does the same thing as jQuery's inArray:

Array.prototype.contains = function(value) {
  for (var i = 0; i < this.length; i++) {
    if (this[i] === value)
      return true;
  }
  return false;
}

and then use it like this:

if (!groups[groupName].contains(myArray[i].color)) {
  groups[groupName].push(myArray[i].color);
}

Note that in either case you are going to slow things down a bit due to all the extra iteration, so if you don't need to avoid duplicate color entries in the result arrays I would recommend avoiding this extra code. There

Start by creating a mapping of group names to values. Then transform into your desired format.

var myArray = [
    {group: "one", color: "red"},
    {group: "two", color: "blue"},
    {group: "one", color: "green"},
    {group: "one", color: "black"}
];

var group_to_values = myArray.reduce(function (obj, item) {
    obj[item.group] = obj[item.group] || [];
    obj[item.group].push(item.color);
    return obj;
}, {});

var groups = Object.keys(group_to_values).map(function (key) {
    return {group: key, color: group_to_values[key]};
});

var pre = document.createElement("pre");
pre.innerHTML = "groups:\n\n" + JSON.stringify(groups, null, 4);
document.body.appendChild(pre);

Using Array instance methods such as reduce and map gives you powerful higher-level constructs that can save you a lot of the pain of looping manually.

Use lodash's groupby method

Creates an object composed of keys generated from the results of running each element of collection thru iteratee. The order of grouped values is determined by the order they occur in collection. The corresponding value of each key is an array of elements responsible for generating the key. The iteratee is invoked with one argument: (value).

So with lodash you can get what you want in a single line. See below

let myArray = [
  {group: "one", color: "red"},
  {group: "two", color: "blue"},
  {group: "one", color: "green"},
  {group: "one", color: "black"},
]
let grouppedArray=_.groupBy(myArray,'group')
console.log(grouppedArray)

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

One option is:

var res = myArray.reduce(function(groups, currentValue) {
    if ( groups.indexOf(currentValue.group) === -1 ) {
      groups.push(currentValue.group);
    }
    return groups;
}, []).map(function(group) {
    return {
        group: group,
        color: myArray.filter(function(_el) {
          return _el.group === group;
        }).map(function(_el) { return _el.color; })
    }
});

http://jsfiddle.net/dvgwodxq/

Beside the given approaches with a two pass approach, you could take a single loop approach by pushing the group if a new group is found.

var array = [{ group: "one", color: "red" }, { group: "two", color: "blue" }, { group: "one", color: "green" }, { group: "one", color: "black" }],
    groups = Object.create(null),
    grouped = [];

array.forEach(function (o) {
    if (!groups[o.group]) {
        groups[o.group] = [];
        grouped.push({ group: o.group, color: groups[o.group] });
    }
    groups[o.group].push(o.color);
});

console.log(grouped);

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

This version takes advantage that object keys are unique. We process the original array and collect the colors by group in a new object. Then create new objects from that group -> color array map.

var myArray = [{
      group: "one",
      color: "red"
    }, {
      group: "two",
      color: "blue"
    }, {
      group: "one",
      color: "green"
    }, {
      group: "one",
      color: "black"
    }];

    //new object with keys as group and
    //color array as value
    var newArray = {};

    //iterate through each element of array
    myArray.forEach(function(val) {
      var curr = newArray[val.group]

      //if array key doesnt exist, init with empty array
      if (!curr) {
        newArray[val.group] = [];
      }

      //append color to this key
      newArray[val.group].push(val.color);
    });

    //remove elements from previous array
    myArray.length = 0;

    //replace elements with new objects made of
    //key value pairs from our created object
    for (var key in newArray) {
      myArray.push({
        'group': key,
        'color': newArray[key]
      });
    }

Please note that this does not take into account duplicate colors of the same group, so it is possible to have multiple of the same color in the array for a single group.

Another option is using reduce() and new Map() to group the array. Use Spread syntax to convert set object into an array.

var myArray = [{"group":"one","color":"red"},{"group":"two","color":"blue"},{"group":"one","color":"green"},{"group":"one","color":"black"}]

var result = [...myArray.reduce((c, {group,color}) => {
  if (!c.has(group)) c.set(group, {group,color: []});
  c.get(group).color.push(color);
  return c;
}, new Map()).values()];

console.log(result);

var array = [{
      id: "123",
      name: "aaaaaaaa"
    }, {
      id: "123",
      name: "aaaaaaaa"
    }, {
      id: '456',
      name: 'bbbbbbbbbb'
    }, {
      id: '789',
      name: 'ccccccccc'
    }, {
      id: '789',
      name: 'ccccccccc'
    }, {
      id: '098',
      name: 'dddddddddddd'
    }];
//if you want to group this array
group(array, key) {
  console.log(array);
  let finalArray = [];
  array.forEach(function(element) {
    var newArray = [];
    array.forEach(function(element1) {
      if (element[key] == element1[key]) {
          newArray.push(element)
      }
    });
    if (!(finalArray.some(arrVal => newArray[0][key] == arrVal[0][key]))) {
        finalArray.push(newArray);
    }
  });
  return finalArray
}
//and call this function
groupArray(arr, key) {
  console.log(this.group(arr, key))
}

You can do something like this:

function convert(items) {
    var result = [];

    items.forEach(function (element) {
        var existingElement = result.filter(function (item) {
            return item.group === element.group;
        })[0];

        if (existingElement) {
            existingElement.color.push(element.color);
        } else {
            element.color = [element.color];
            result.push(element);
        }

    });

    return result;
}

Using Array's reduce and findIndex methods, this can be achieved.

var myArray = [{
  group: "one",
  color: "red"
}, {
  group: "two",
  color: "blue"
}, {
  group: "one",
  color: "green"
}, {
  group: "one",
  color: "black"
}];

var transformedArray = myArray.reduce((acc, arr) => {
  var index = acc.findIndex(function(element) {
    return element.group === arr.group;
  });
  if (index === -1) {
    return acc.push({
      group: arr.group,
      color: [arr.color]
    });
  }
  
  acc[index].color.push(arr.color);
  return acc;
}, []);

console.log(transformedArray);

By using reduce function, array is iterator and the new values are stored in acc (accumulating) parameter. To check if the object with given group already exists we can use findIndex function.

If findIndex() return -1, the value does not exist, so add the array in the acc parameter.

If findIndex() return index, then update the index with the arr values.

You can extend array functionality with the next:

Array.prototype.groupBy = function(prop) {
  var result = this.reduce(function (groups, item) {
      const val = item[prop];
      groups[val] = groups[val] || [];
      groups[val].push(item);
      return groups;
  }, {});
  return Object.keys(result).map(function(key) {
      return result[key];
  });
};

Usage example:

/* re-usable function */
Array.prototype.groupBy = function(prop) {
  var result = this.reduce(function (groups, item) {
      const val = item[prop];
      groups[val] = groups[val] || [];
      groups[val].push(item);
      return groups;
  }, {});
  return Object.keys(result).map(function(key) {
      return result[key];
  });
};

var myArray = [
  {group: "one", color: "red"},
  {group: "two", color: "blue"},
  {group: "one", color: "green"},
  {group: "one", color: "black"}
]

console.log(myArray.groupBy('group'));

Credits: @Wahinya Brian

My approach with a reducer:

myArray = [
  {group: "one", color: "red"},
  {group: "two", color: "blue"},
  {group: "one", color: "green"},
  {group: "one", color: "black"}
]

console.log(myArray.reduce( (acc, curr) => {
  const itemExists = acc.find(item => curr.group === item.group)
  if(itemExists){
    itemExists.color = [...itemExists.color, curr.color]
  }else{
    acc.push({group: curr.group, color: [curr.color]})
  }
  return acc;
}, []))

This gives you unique colors, if you do not want duplicate values for color

var arr = [
  {group: "one", color: "red"},
  {group: "two", color: "blue"},
  {group: "one", color: "red"},
  {group: "two", color: "blue"},
  {group: "one", color: "green"},
  {group: "one", color: "black"}
]

var arra = [...new Set(arr.map(x => x.group))]

let reformattedArray = arra.map(obj => {
   let rObj = {}
   rObj['color'] = [...new Set(arr.map(x => x.group == obj ? x.color:false ))]
       .filter(x => x != false)
   rObj['group'] = obj
   return rObj
})
console.log(reformattedArray)

this repo offers solutions in lodash and alternatives in native Js, you can find how to implement groupby. https://github.com/you-dont-need/You-Dont-Need-Lodash-Underscore#_groupby

Using ES6, this can be done quite nicely using .reduce() with a Map as the accumulator, and then using Array.from() with its mapping function to map each grouped map-entry to an object:

const arr = [{"group":"one","color":"red"},{"group":"two","color":"blue"},{"group":"one","color":"green"},{"group":"one","color":"black"}];

const res = Array.from(arr.reduce((m, {group, color}) => 
    m.set(group, [...(m.get(group) || []), color]), new Map
  ), ([group, color]) => ({group, color})
);

console.log(res);

If you have additional properties in your objects other than just group and color, you can take a more general approach by setting a grouped object as the map's values like so:

const arr = [{"group":"one","color":"red"},{"group":"two","color":"blue"},{"group":"one","color":"green"},{"group":"one","color":"black"}];

const groupAndMerge = (arr, groupBy, mergeInto) => 
  Array.from(arr.reduce((m, o) => {
    const curr = m.get(o[groupBy]);
    return m.set(o[groupBy], {...o, [mergeInto]: [...(curr && curr[mergeInto] || []), o[mergeInto]]});
  }, new Map).values());

console.log(groupAndMerge(arr, 'group', 'color'));

If you can support optional chaining, you can simplify the above method to the following:

const arr = [{"group":"one","color":"red"},{"group":"two","color":"blue"},{"group":"one","color":"green"},{"group":"one","color":"black"}];
const groupAndMerge = (arr, groupBy, mergeWith) =>
  Array.from(arr.reduce((m, o) => m.set(o[groupBy], {...o, [mergeWith]: [...(m.get(o[groupBy])?.[mergeWith] || []), o[mergeWith]]}), new Map).values());

console.log(groupAndMerge(arr, 'group', 'color'));

Try (h={})

myArray.forEach(x=> h[x.group]= (h[x.group]||[]).concat(x.color) );
myArray = Object.keys(h).map(k=> ({group:k, color:h[k]}))

let myArray = [
  {group: "one", color: "red"},
  {group: "two", color: "blue"},
  {group: "one", color: "green"},
  {group: "one", color: "black"},
];

let h={};

myArray.forEach(x=> h[x.group]= (h[x.group]||[]).concat(x.color) );
myArray = Object.keys(h).map(k=> ({group:k, color:h[k]}))

console.log(myArray);