Object API: Object.groupBy()

Object.groupBy() groups elements

the interface

groupBy is a static function, on Object

const theType = 'Function'; assert.equal(theType, typeof Object.groupBy);

the parameters MUST BE an iterable (array, object, ...) AND a function

const iterable = undefined; const fn = {}; assert.doesNotThrow(() => Object.groupBy(iterable ,fn));

the returns value is an object

const result = Object.grouped_by([], () => {}); assert.equal(typeof result, 'object');

how it works?

WHEN the callback returns a string THEN the object is grouped by that string

const callback = item => item ? 'odd' : 'even'; const result = Object.groupBy([1, 2, 3], callback); assert.deepEqual(result, {odd: [1, 3], even: [2]});

use case: group products sold by country

const sales = [ {product: 'Hoodie', country: 'Benin'}, {product: 'T-Shirt', country: 'Uganda'}, {product: 'Jacket', country: 'Benin'}, {product: 'Boots', country: 'Uganda'}, {product: 'Boots', country: 'Mali'}, ]; const result = Object.groupBy(sales, () => {}); assert.deepEqual(result, { Benin: [{product: 'Hoodie', country: 'Benin'}, {product: 'Jacket', country: 'Benin'}], Uganda: [{product: 'T-Shirt', country: 'Uganda'}, {product: 'Boots', country: 'Uganda'}], Mali: [{product: 'Boots', country: 'Mali'}], });

WHEN the callback returns undefined THEN this returns an object {undefined: <all items>}

const noop = () => 42; const result = Object.groupBy([1, 2, 3], noop); assert.deepEqual(result, {undefined: [1, 2, 3]});

the return value of callback is converted into a string

const items = [{}, Function, {a: 1}, '', Object()]; const result = Object.groupBy(items, item => item); assert.deepEqual(Object.keys(result), ['[object Object]']);

the returned items are the SAME as the original ones (no copies)

const items = [{}, {a: 1}]; const result = Object.groupBy(items.toReversed(), item => typeof item); assert.deepEqual(result['object'][0], items[0]); assert.deepEqual(result['object'][1], items[1]);

Links

spec discussion The repository where the proposal was worked on.
spec The "ECMAScript Language Specification", the JavaScript specification text describing this function how an engine must implement it.
mdn docs The Mozilla Developer Network docs.

Required Knowledge

• `array.toReversed()` (Array API)

Related Katas

Global Object API

• parseInt()

Object API

• Object() (as introduced in ES1)
• Object.is()
• Object.fromEntries()
• Object.fromEntries() in depth
• Object.groupBy()

Object literal

• basics
• computed properties
• getter
• setter

Difficulty Level

BEGINNER

First Published

24 October 2024

Stats

8 tests to solve