Keyed Collections

`Map` object type

const mp = new Map([[key1, value1], [key2, value2]]);

const mp = new Map();
mp.set(key1, value1);

mp.get(key1)

collection of key-value pairs, a better object 🎉
any data type as key, not only String or Symbol 🎉
as keys NaN is considered equal to itself, +0 and -0 are considered equal to each other 🎉
iterable with value [key, value], e.g. can use for…of instead of for…in, no prototype leakage anymore 🎉
iteration order is insertion order of keys (for objects is only since ES6) 🎉
has size property 🎉
has iteration methods Map.prototype.keys(), Map.prototype.values(), Map.prototype.forEach() 🎉
lookup operations are faster 🎉
beware: don’t use dot / bracket notation, since properties are set on map as object properties instead of map properties ⚠️

Map object, but only objects as keys
keys are weekly referenced, i.e. garbage collection isn’t prevented incase there’s no other reference to the object
good to prevent memory leaking
not enumerable, because garbage collection is not determined

const st = new Set([1, 2, 3, 3, 3]);

const st = new Set();
st.add([1, 2, 3, 3, 3]);

collection of values, like array
values must be unique, duplicates get discarded, i.e. can make array unique by converting to set and back ❗️
no access to value at index, because no actionable order due to duplicate filter
can delete elements by value since uniquely identifiable, in array needs to splice by index
as values NaN is considered equal to itself, +0 and -0 are considered equal to each other 🎉
has size property 🎉
lookup operations are faster, e.g. Set.prototype.has() is faster than Array.prototype.indexOf() 🎉
beware: don’t use dot / bracket notation, since properties are set on set as object properties instead of set properties ⚠️