Map, Set Constructors

Maps and Sets (along with WeakMaps and WeakSets) were created to help solve issues with objects.

Map()

The Map object holds unique key/value pairs. Any value (both objects and primitive values) may be used as either a key or a value. Click here for a quick look at the properties and methods.

creation

To create a new map, use new Map([iterable]). Iterable can be an array or any iterable object whose elements are key/value pairs. You can even throw a tuple in there!

const tuple = [['carlos', 'piano'],['nicolas', 'saxophone'], ['william', 'violin']];
const map = new Map(tuple);
console.log(map.get('carlos'));

reliable orderOne switch from objects is that Map objects iterate elements in insertion order, reliably. When using a for/in loop, you can't be 100% sure your keys will be in order, as the Objects aren't required to iterate in order. Seems like they do most of the time, but Maps eliminate that doubt.

directly iterableUnlike Objects, you can iterate directly on a Map. Very sweet!

const boys = {captain: 'greg', geek: 'cameron', funnyboy: 'aidan', president:'nicolas'};
const classBoys = new Map(Object.entries(boys));
classBoys.forEach(boy => console.log(boy));

size propertyUnlike Objects, you can get the length of elements in your map easily through .size().

const boys = {captain: 'greg', geek: 'cameron', funnyboy: 'aidan', president:'nicolas'};
const classBoys = new Map(Object.entries(boys));
console.log('number of boys in class '+classBoys.size);

Set()

A Set is a unique list of values. One common use for a Set is to convert your array to a Set in order to remove duplicate values. Like Maps, they can also consist of primitive values or object references. See here for properties & methods.

creation

const girls = ['julia', 'sophia', 'gillian', 'belicia', 'sophia', 'sarah', 'natalie'];
const set = new Set(girls);
console.log('number of unique girl names in class '+set.size);
 set.forEach(girl => console.log(girl));

array differences

Here are the key things to remember (read more here):

• Sets will toss out any duplicates
• Removing elements from Set is easier since you don't need the index
• Set doesn't support access by index number (i.e. girls[0])
• Arrays will be faster for iteration & random access

Sets are also potentially better when you want to perform operations on a dataset, such as union, difference, or intersection.

union

Combine all elements from 2 set into a new set.

const school =  new Set(['julia', 'sophia', 'gillian', 'belicia','sarah', 'natalie']);
const girlScouts = new Set(['sophia', 'kate', 'helen', 'julia']);

const allGirls = new Set([...school, ...girlScouts]);
console.log('union of all girls: '+ Array.from(allGirls));
 

intersection

Retrieve elements that are contained in both sets.

const school =  new Set(['julia', 'sophia', 'gillian', 'belicia','sarah', 'natalie']);
const girlScouts = new Set(['sophia', 'kate', 'helen', 'julia']);
const schoolScouts = new Set([...school].filter(girl => girlScouts.has(girl)));
console.log('girl Scouts in class: '+ Array.from(schoolScouts));
 

difference

Elements are are in first set but not second set.

const school =  new Set(['julia', 'sophia', 'gillian', 'belicia','sarah', 'natalie']);
const girlScouts = new Set(['sophia', 'kate', 'helen', 'julia']);
const nonScouts = new Set([...school].filter(girl => !girlScouts.has(girl)));
console.log('girls not in Scouts: '+ Array.from(nonScouts));