Array.prototype.groupByToMap()

The groupByToMap() method groups the elements of the calling array using the values returned by a provided testing function. The final returned Map uses the unique values from the test function as keys, which can be used to get the array of elements in each group.

The method is primarily useful when grouping elements that are associated with an object, and in particular when that object might change over time. If the object is invariant, you might instead represent it using a string, and group elements with Array.prototype.groupBy().

callbackFn

Function to execute on each element in the array.

The function is called with the following arguments:

element

The current element in the array.

index

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

array

The array that groupByToMap() was called on.

The value (object or primitive) returned from the callback indicates the group of the current element.

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.

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

TypeError

The specified callback function is not callable.

The groupByToMap() method executes callbackFn once for each index of the array. The callback function returns a value indicating the group of the associated element. The values returned by callbackFn are used as keys for the Map returned by groupByToMap(). 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 array are the same (not deep copies). Changing the internal structure of the elements will be reflected in both the original array 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 Array.prototype.groupBy(), you must maintain the mapping between the original object and its representation as the object changes.

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 groupByToMap(), it will be used as the this value inside each invocation of the callbackFn. If it is not provided, then undefined is used.

The groupByToMap() 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 groupByToMap() are set before the first invocation of callbackFn. Therefore:

• callbackFn will not visit any elements added to the array after the call to groupByToMap() 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 groupByToMap() 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: 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 groupByToMap() 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 = inventory.groupByToMap( ({ quantity }) => quantity < 6 ? restock : sufficient );
console.log(result.get(restock));
// expected output: Array [Object { 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));
// expected output: Array [Object { 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));
// expected output: undefined

• Array.prototype.groupBy()
• Polyfill of Array.prototype.groupBy in core-js