JS Set & Map

Question 1

Set

Answer 1

The Set object lets you store unique values of any type, whether primitive values or object references.

Set objects are collections of values. A value in the Set may only occur once; it is unique in the Set’s collection. You can iterate through the elements of a set in insertion order.

Question 2

Set.prototype.size

Answer 2

Returns the number of values in the Set object.

Question 3

Set.prototype.add(value)

Answer 3

Inserts a new element with a specified value in to a Set object, if there isn’t an element with the same value already in the Set.

Question 4

Set.prototype.clear()

Map.prototype.clear()

Answer 4

Removes all elements from the Set || Map object.

Question 5

Set.prototype.delete(value)

Answer 5

Removes the element associated to the value and returns a boolean asserting whether an element was successfully removed or not. Set.prototype.has(value) will return false afterwards.

Question 6

Set.prototype.has(value)

Answer 6

Returns a boolean asserting whether an element is present with the given value in the Set object or not.

Question 7

What are the main methods of a Set?

Answer 7

.add(value)
.delete(value)
.has(value)

.clear()

Question 8

Set.prototype.forEach(callbackFn[, thisArg])

Answer 8

Calls callbackFn once for each value present in the Set object, in insertion order. If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackFn.

Question 9

Set.prototype.values()

Answer 9

Returns a new iterator object that yields the values for each element in the Set object in insertion order.

Question 10

Map

Answer 10

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.

Question 11

Objects vs. Maps

Answer 11

A Map does not contain any keys by default

A Maps keys can be any value (functions, objects, etc)
while an object has to be String or Symbol

The keys in Map are ordered in a simple, straightforward way

The number of items in a Map is easily retrieved from its size property.

A Map is an iterable, so it can be directly iterated.

A Map is better if there are frequent removals or additions

Question 12

Map.prototype.size

Answer 12

Returns the number of key/value pairs in the Map object.

Question 13

Map.prototype.get(key)

Answer 13

Returns the value associated to the key, or undefined if there is none.

Question 14

Map.prototype.has(key)

Answer 14

Returns a boolean asserting whether a value has been associated to the key in the Map object or not.

Question 15

Map.prototype.set(key, value)

Answer 15

Sets the value for the key in the Map object. Returns the Map object.

Question 16

Map.prototype.keys()

Answer 16

Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.

Question 17

Map.prototype.values()

Answer 17

Returns a new Iterator object that contains the values for each element in the Map object in insertion order.

Question 18

Map.prototype.entries()

Answer 18

Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order.

Question 19

Map.prototype.forEach(callbackFn[, thisArg])

Answer 19

Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback.

Question 20

Iterate a Map with a for…of loop

Answer 20

for (const [key, value] of myMap) {
console.log(${key} = ${value});
}

Question 21

Iterate a map with .forEach()

Answer 21

myMap.forEach((value, key) => {
console.log(${key} = ${value});
});

Question 22

How would you merge Maps?

Answer 22

const merged = new Map([…first, …second]);

const merged = new Map([…first, …second, [1, ‘eins’]]);

Question 23

iterate of a set with for…of

Answer 23

for (const item of mySet1) {
console.log(item);
}

Question 24

iterate over a Set with .forEach()

Answer 24

mySet2.forEach((value) => {
console.log(value);
});

Question 25

How can you remove duplicate elements in an Array?

Answer 25

arraySet = …new Set(array);

Question 26

How can you remove duplicates from a string?

Answer 26

(case-sensitive)

new Set(“Firefox”) // Set(7) { “F”, “i”, “r”, “e”, “f”, “o”, “x” }

new Set(“firefox”) // Set(6) { “f”, “i”, “r”, “e”, “o”, “x” }

Question 27

Use Set to ensure the uniqueness of a list

Answer 27

const array = Array
  .from(document.querySelectorAll('[id]'))
  .map((e) => e.id);

const set = new Set(array);
console.assert(set.size === array.length);

Question 28

What are the main methods of Map?

Answer 28

.set(key, value)
.delete(key)
.has(key)
.get(key)

.clear()