Array.prototype.groupBy()

The groupBy() method groups the elements of the calling array according to the string values returned by a provided testing function. The returned object has separate properties for each group, containing arrays with the elements in the group.

This method should be used when group names can be represented by strings. If you need to group elements using a key that is some arbitrary value, use Array.prototype.groupByToMap() instead.

callbackFn

Function to execute on each element in the array

The function is called with the following arguments:

element

The value of the current element in the array.

index

The index (position) of the current element in the array.

array

The array that groupBy() was called on.

The object returned from the callback indicates the group of the current element. It must be possible to coerce this returned callback value into a string (which will then be used as a property name in the final returned object).

thisArg Optional

Object to use as this inside callbackFn.

The argument is ignored in arrow functions, as they have their own lexical scope that will be used instead. Otherwise, if thisArg not specified, then either the this of the executing scope is used, or undefined if the function is called in strict mode.

An Object with properties for all groups, each assigned to an array containing the elements of the associated group. The value is an object that does not inherit from Object.prototype.

TypeError

The specified callback function is not callable.

The groupBy() method executes the callbackFn function once for each index of the array, returning a string (or value that can be coerced to a string) indicating the group of the element. A new property and array is created in the result object for each unique group name that is returned by the callback. Each element is added to the array in the property that corresponds to its group.

Note that the returned object references the same elements as the original array (not deep copies). Changing the internal structure of these elements will be reflected in both the original array and the returned object.

callbackFn is called with the value of the current element, the current index, and the array itself. While groups often depend only on the current element, it is possible to implement grouping strategies based on the values of other elements in the array.

callbackFn is invoked for every index of the array, not just those with assigned values. This means it may be less efficient for sparse arrays, compared to methods that only visit assigned values.

If a thisArg parameter is provided to groupBy(), it will be used as the this value inside each invocation of the callbackFn. If it is not provided, then undefined is used.

The groupBy() method does not mutate the array on which it is called, but the function provided to callbackFn can. Note however that the elements processed by groupBy() are set before the first invocation of callbackFn. Therefore:

• callbackFn will not visit any elements added to the array after the call to groupBy() begins.
• Elements that are assigned to indexes already visited, or to indexes outside the range, will not be visited by callbackFn.
• If an existing, yet-unvisited element of the array is changed by callbackFn, its value passed to the callbackFn will be the value at the time groupBy() visits that element's index.
• Elements that are deleted are still visited.

First we define an array containing objects representing an inventory of different foodstuffs. Each food has a type and a quantity.

const inventory = [
  { name: 'asparagus', type: 'vegetables', quantity: 5 },
  { name: 'bananas',  type: 'fruit', quantity: 0 },
  { name: 'goat', type: 'meat', quantity: 23 },
  { name: 'cherries', type: 'fruit', quantity: 5 },
  { name: 'fish', type: 'meat', quantity: 22 }
];

The code below groups the elements by the value of their type property.

let result = inventory.groupBy( ({ type }) => type );

/* Result is:
{ 
  vegetables: [
    { name: 'asparagus', type: 'vegetables', quantity: 5 }, 
  ],
  fruit: [
    { name: "bananas", type: "fruit", quantity: 0 },
    { name: "cherries", type: "fruit", quantity: 5 }
  ], 
  meat: [
    { name: "goat", type: "meat", quantity: 23 },
    { name: "fish", type: "meat", quantity: 22 }
  ] 
}
*/

The arrow function just returns the type of each array element each time it is called. Note that the function argument { type } is a basic example of object destructuring syntax for function arguments. This unpacks the type property of an object passed as a parameter, and assigns it to a variable named type in the body of the function. This is a very succinct way to access the relevant values of elements within a function.

We can also create groups inferred from values in one or more properties of the elements. Below is a very similar example that puts the items into ok or restock groups based on the value of the quantity field.

function myCallback( { quantity } ) {
  return quantity > 5 ? 'ok' : 'restock';
}

result = inventory.groupBy( myCallback );

/* Result is:
{ 
  restock: [
    { name: "asparagus", type: "vegetables", quantity: 5 },
    { name: "bananas", type: "fruit", quantity: 0 },
    { name: "cherries", type: "fruit", quantity: 5 }
  ], 
  ok: [
    { name: "goat", type: "meat", quantity: 23 },
    { name: "fish", type: "meat", quantity: 22 }
  ] 
}
*/

• Array.prototype.groupByToMap() – Group an array into a map, using any kind of object as a key or value.
• Polyfill of Array.prototype.groupBy in core-js