items

An iterable (such as an Array) whose elements will be grouped.

callbackFn

A function to execute for each element in the iterable. It should return a value (object or primitive) indicating the group of the current element. The function is called with the following arguments:

element

The current element being processed.

index

The index of the current element being processed.

A Map object with keys for each group, each assigned to an array containing the elements of the associated group.

Map.groupBy() calls a provided callbackFn function once for each element in an iterable. The callback function should return a value indicating the group of the associated element. The values returned by callbackFn are used as keys for the Map returned by Map.groupBy(). Each key has an associated array containing all the elements for which the callback returned the same value.

The elements in the returned Map and the original iterable are the same (not deep copies). Changing the internal structure of the elements will be reflected in both the original iterable and the returned Map.

This method is useful when you need to group information that is related to a particular object that might potentially change over time. This is because even if the object is modified, it will continue to work as a key to the returned Map. If you instead create a string representation for the object and use that as a grouping key in Object.groupBy(), you must maintain the mapping between the original object and its representation as the object changes.

Map.groupBy does not read the value of this. It can be called on any object and a new Map instance will be returned.

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: 9 },
  { name: "bananas", type: "fruit", quantity: 5 },
  { name: "goat", type: "meat", quantity: 23 },
  { name: "cherries", type: "fruit", quantity: 12 },
  { name: "fish", type: "meat", quantity: 22 },
];

The code below uses Map.groupBy() with an arrow function that returns the object keys named restock or sufficient, depending on whether the element has quantity < 6. The returned result object is a Map so we need to call get() with the key to obtain the array.

const restock = { restock: true };
const sufficient = { restock: false };
const result = Map.groupBy(inventory, ({ quantity }) =>
  quantity < 6 ? restock : sufficient,
);
console.log(result.get(restock));
// [{ name: "bananas", type: "fruit", quantity: 5 }]

Note that the function argument { quantity } is a basic example of object destructuring syntax for function arguments. This unpacks the quantity property of an object passed as a parameter, and assigns it to a variable named quantity in the body of the function. This is a very succinct way to access the relevant values of elements within a function.

The key to a Map can be modified and still used. However you can't recreate the key and still use it. For this reason it is important that anything that needs to use the map keeps a reference to its keys.

// The key can be modified and still used
restock["fast"] = true;
console.log(result.get(restock));
// [{ name: "bananas", type: "fruit", quantity: 5 }]

// A new key can't be used, even if it has the same structure!
const restock2 = { restock: true };
console.log(result.get(restock2)); // undefined

• Polyfill of Map.groupBy in core-js
• Indexed collections guide
• Array.prototype.reduce()
• Map()
• Object.groupBy()