ES6: Discovering Sets, WeakSets, Maps and WeakMaps

This article is an extract from my book: The Complete Guide to Modern JavaScript.

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What is a Set?

A Set is an Object where we can store unique values of any type.

// create our set
const family = new Set();

// add values to it
family.add("Dad");
console.log(family);
// Set [ "Dad" ]

family.add("Mom");
console.log(family);
// Set [ "Dad", "Mom" ]

family.add("Son");
console.log(family);
// Set [ "Dad", "Mom", "Son" ]

family.add("Dad");
console.log(family);
// Set [ "Dad", "Mom", "Son" ]

As you can see, at the end we tried to add "Dad" again,but the Set still remained the same because a Set can only take unique values.

Let's continue using the same Set and see what methods we can use on it.

family.size;
// 3
family.keys();
// SetIterator {"Dad", "Mom", "Son"}
family.entries();
// SetIterator {"Dad", "Mom", "Son"}
family.values();
// SetIterator {"Dad", "Mom", "Son"}
family.delete("Dad");
family;
// Set [ "Mom", "Son" ]
family.clear;
family;
// Set []

As you can see a Set has a size property and we can delete an item from it or use clear to delete all the items from it.

We can also notice that a Set does not have keys so when we call .keys() we get the same as calling .values() or .entries().

 

Loop over a Set

We have two ways of iterating over a Set: using .next() or using a for of loop.

// using `.next()`
const iterator = family.values();
iterator.next();
// Object { value: "Dad", done: false }
iterator.next();
// Object { value: "Mom", done: false }


// using a `for of` loop
for(const person of family) {
  console.log(person);
}
// Dad
// Mom
// Son

 

Remove duplicates from an array

We can use a Set to remove duplicates from an Array since we know it can only hold unique values.

const myArray = ["dad", "mom", "son", "dad", "mom", "daughter"];

const set = new Set(myArray);
console.log(set);
// Set [ "dad", "mom", "son", "daughter" ]
// transform the `Set` into an Array
const uniqueArray = Array.from(set);
console.log(uniqueArray);
// Array [ "dad", "mom", "son", "daughter" ]

// write the same but in a single line
const uniqueArray = Array.from(new Set(myArray));
// Array [ "dad", "mom", "son", "daughter" ]

As you can see the new array only contains the unique values from the original array.

 

What is a WeakSet?

A WeakSet is similar to a Set but it can only contain Objects.

let dad = {name: "Daddy", age: 50};
let mom = {name: "Mummy", age: 45};

const family = new WeakSet([dad,mom]);

for(const person of family){
  console.log(person);
}
// TypeError: family is not iterable

We created our new WeakSet but when we tried to use a for of loop it did not work, we can't iterate over a WeakSet.

A WeakSet cleans itself up after we delete an element from it.

dad = null;
family;
// WeakSet [ {…}, {…} ]

// wait a few seconds
family;
// WeakSet [ {…} ]

As you can see after a few seconds dad was removed and garbage collected. That happened because the reference to it was lost when we set the value to null.

 

What is a Map?

A Map is similar to a Set, but they have key and value pairs.

const family = new Map();

family.set("Dad", 40);
family.set("Mom", 50);
family.set("Son", 20);

family;
// Map { Dad → 40, Mom → 50, Son → 20 }
family.size;
// 3

family.forEach((key,val) => console.log(val,key));
// Dad 40
// Mom 50
// Son 20

for(const [key,val] of family){
  console.log(key,val);
}
// Dad 40
// Mom 50
// Son 20

If you remember, we could iterate over a Set only with a for of loop, while we can iterate over a Map with both a for of and a forEach loop.

 

What is a WeakMap?

A WeakMap is a collection of key/value pairs and similarly to a WeakSet, even in a WeakMap the keys are weakly referenced, which means that when the reference is lost, the value will be removed from the WeakMap and garbage collected.

A WeakMap is not enumerable therefore we cannot loop over it.

let dad = { name: "Daddy" };
let mom = { name: "Mommy" };

const myMap = new Map();
const myWeakMap = new WeakMap();

myMap.set(dad);
myWeakMap.set(mom);

dad = null;
mom = null;

myMap;
// Map(1) {{…}}
myWeakMap;
// WeakMap {}

As you can see mom was garbage collected after we set the its value to null whilst dad still remains inside our Map.

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