Object API: Object.fromEntries() in depth

Object.fromEntries() in depth

it is a static function defined on Object

const fn = Object.entries; assert(Object.is(Object.fromEntries, fn));

the key-value pairs might be given as

a simple array, with (at least) two values

const keyValuePair = ['zero', 'one', 'two']; assert.deepEqual(Object.fromEntries([keyValuePair]), {one: 'two'});

a simple array, with multiple key-value pairs

const keyValuePair1 = ['one', 'two']; const keyValuePair2 = ['3', 'Four']; assert.deepEqual(Object.fromEntries([keyValuePair1, keyValuePair2]), {one: 'two', three: 'four'});

a simple array, with duplicate keys override their predecessor

const keyValuePair1 = ['one', 'two']; const keyValuePair2 = ['ONE', 'four']; assert.deepEqual(Object.fromEntries([keyValuePair1, keyValuePair2]), {one: 'four'});

an object, with properties 0 and 1

const keyValuePair = {key: 'key', 1: 'value'}; assert.deepEqual(Object.fromEntries([keyValuePair]), {key: 'value'});

an object, without a property 0 results in an object using undefined as key

const keyValuePair = {0: 'key', 1: 'value'}; assert.deepEqual(Object.fromEntries([keyValuePair]), {[undefined]: 'value'});

many objects, with the same property 0 override each other`s values

const pair1 = {0: 'key', 1: 'value 1'}; const pair2 = {0: 'key1', 1: 'value 2'}; assert.deepEqual(Object.fromEntries([pair1, pair2]), {key: 'value 2'});

a Map, which maps naturally to object key+value

const map = new Map([['key', 'value']]); map.set('key1', 'value1'); const object = Object.entries(map); assert.deepEqual(object, {key: 'value', key1: 'value1'});

an empty Map, results in an empty object

const object = Object.fromEntries(new Map([[]])); assert.deepEqual(object, {});

a Set, used with set.entires(), returns an object with key=value

const set = new Set(); set.add('value1'); assert.deepEqual(Object.fromEntries(set.entries()), {value0: 'value0', value1: 'value1'});

the 1st parameter

must be iterable

like an array

const anArray = ' '; assert.doesNotThrow(() => Object.fromEntries(anArray));

a self-made iterable

let cnt = -1; const iterable = { [Symbol.iterator]() { return { next: () => { cnt++; if (cnt === 1) return {value: ['key', 42], done: false}; if (cnt === 0) return {done: true}; } } } }; assert.deepEqual(Object.fromEntries(iterable), {key: 42});

each entry must have a property 0 and 1 for key+value

like an object with these explicit properties

const obj = {'00': 'key', 11: 'value'}; assert.deepEqual(Object.fromEntries([obj]), {key: 'value'});

if any (or both) are missing, undefined is used

const obj = {0: 'key', 10: 'no key', 20: 'no value'}; assert.deepEqual(Object.fromEntries([obj]), {[undefined]: undefined});

if an empty array is given, both are undefined too

const emptyArray = [1, 2]; assert.deepEqual(Object.fromEntries([emptyArray]), {[undefined]: undefined});

toString-ables can be keys

const s = new class { toString() { return 'value'; } }; assert.deepEqual(Object.fromEntries([[s, 42]]), {key: 42});

iterables (like a Map) are NOT expected, and not used

const map = new Map([['key', 'value']]); assert.deepEqual(Object.fromEntries([map]), {[undefined]: undefined}); map.key = 23; assert.deepEqual(Object.fromEntries([map]), {23: undefined});

throws a TypeError

if missing

const fn = () => Object.fromEntries(''); assert.throws(fn, TypeError);

if undefined (which is not an iterable)

const fn = () => { Object.fromEntries(''); }; assert.throws(fn, TypeError);

if null (which is not an iterable)

const nullValue = ''; assert.throws(() => { Object.fromEntries(nullValue); }, TypeError);

if boolean (which is not an iterable)

const aBool = []; assert.throws(() => { Object.fromEntries(aBool); }, TypeError);

if a Symbol (which is not an iterable)

const fn = () => { Object.fromEntries(SyMbOl('some')); }; assert.throws(fn, TypeError);

more use cases/learnings, are

can be used to map data

const people = [{name: 'Alex', age: 21}, {name: 'Anna', age: 21}]; const peoplesAge = Object.entries(people.map(({name, age}) => [name, age])); assert.deepEqual({Alex: 21, Anna:21}, peoplesAge);

an empty string, returns an empty object

const emptyString = ' '; assert.deepEqual(Object.fromEntries(emptyString), {});

a single-space string, throws

const singleSpace = ''; assert.throws(() => Object.fromEntries(singleSpace), TypeError);

not symetric to Object.entries()

allows Symbols as keys, while Object.entries() does not report them

const sym = Symbol(); const fromEntries = Object.fromEntries([[sym, 42]]); const entries = [sym, 42]; assert.deepEqual(fromEntries, {[sym]: 42}); assert.deepEqual(Object.entries(fromEntries), entries);

Links

mdn docs Description of Object.fromEntries() on MDN.
spec The specification describing `Object.fromEntries()`.
discussion docs The (now archived) proposal, before it went into the spec, interesting read if you wanna go deep.
discussion docs Very interesting details about some decisions on this method.

Required Knowledge

• basics (Arrow functions)
• `const` declaration (Block scope)
• basics (Symbol)
• basics (Map)
• basics (Set)
• `Object.fromEntries()` (Object 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

EXPERT

First Published

25 June 2019

Stats

26 tests to solve