JavaScript Array groupBy method

Array.prototype doesn't have any grouping functionality, so let's add the groupBy() method. Let's say we have the following array example:

  
var movies = [
                 {  
                   "Title": "Inception",
                   "Year": "2010",
                   "Director": "Christopher Nolan"
                },
                {  
                   "Title": "Interstellar",
                   "Year": "2014",
                   "Director": "Christopher Nolan"
                },
                {
                   "Title": "Avatar",
                   "Year": "2009",
                   "Director": "James Cameron"
                },
                {
                   "Title": "The Dark Knight",
                   "Year": "2008",
                   "Director": "Christopher Nolan"
                },
                {  
                   "Title": "Batman Begins",
                   "Year": "2005",
                   "Director": "Christopher Nolan"
                }
];

console.table(movies.groupBy('Director'));

1. var movies = [
2. {
3. "Title": "Inception",
4. "Year": "2010",
5. "Director": "Christopher Nolan"
6. },
7. {
8. "Title": "Interstellar",
9. "Year": "2014",
10. "Director": "Christopher Nolan"
11. },
12. {
13. "Title": "Avatar",
14. "Year": "2009",
15. "Director": "James Cameron"
16. },
17. {
18. "Title": "The Dark Knight",
19. "Year": "2008",
20. "Director": "Christopher Nolan"
21. },
22. {
23. "Title": "Batman Begins",
24. "Year": "2005",
25. "Director": "Christopher Nolan"
26. }
27. ];
29. console.table(movies.groupBy('Director'));

var movies = [
                 {  
                   "Title": "Inception",
                   "Year": "2010",
                   "Director": "Christopher Nolan"
                },
                {  
                   "Title": "Interstellar",
                   "Year": "2014",
                   "Director": "Christopher Nolan"
                },
                {
                   "Title": "Avatar",
                   "Year": "2009",
                   "Director": "James Cameron"
                },
                {
                   "Title": "The Dark Knight",
                   "Year": "2008",
                   "Director": "Christopher Nolan"
                },
                {  
                   "Title": "Batman Begins",
                   "Year": "2005",
                   "Director": "Christopher Nolan"
                }
];

console.table(movies.groupBy('Director'));

The groupBy('Director') method should return 2 groups - 1st "Christopher Nolan" group and 2nd "James Cameron" group. Every group should contain the list of associated movies.

Now let's have a look at the method implementation.

  
Array.prototype.groupBy = function(field){
  let groupedArr = [];
  this.forEach(function(e){
    //look for an existent group
    let group = groupedArr.find(g => g['field'] === e[field]);
    if (group == undefined){
      //add new group if it doesn't exist
      group = {field: e[field], groupList: []};
      groupedArr.push(group);
    }
    
    //add the element to the group
    group.groupList.push(e);
  });
  
  return groupedArr;
}

1. Array.prototype.groupBy = function(field){
2. let groupedArr = [];
3. this.forEach(function(e){
4. //look for an existent group
5. let group = groupedArr.find(g => g['field'] === e[field]);
6. if (group == undefined){
7. //add new group if it doesn't exist
8. group = {field: e[field], groupList: []};
9. groupedArr.push(group);
10. }
12. //add the element to the group
13. group.groupList.push(e);
14. });
16. return groupedArr;
17. }

Array.prototype.groupBy = function(field){
  let groupedArr = [];
  this.forEach(function(e){
    //look for an existent group
    let group = groupedArr.find(g => g['field'] === e[field]);
    if (group == undefined){
      //add new group if it doesn't exist
      group = {field: e[field], groupList: []};
      groupedArr.push(group);
    }
    
    //add the element to the group
    group.groupList.push(e);
  });
  
  return groupedArr;
}

You can try it on JSFiddler:
http://jsfiddle.net/AndrewBuntsev/7syen826/
Don't forget to press F12 to display browser's console