Map and Set

Question 1

What is a map object?

Answer 1

An object with keyed data items but can take keys of any type.
Strings
Numbers
Boolean
Objects
Null
Undefined
Infinity
NaN

Question 2

Create a new map

Answer 2

new Map()

Question 3

Store a value by the key in a map object

Answer 3

Map.prototype.set(key, value)

Question 4

Return the value by the key in a map object, undefined if key doesn’t exist in map.

Answer 4

Map.prototype.get(key)

Question 5

Return true if the key exists in a map object, false otherwise.

Answer 5

Map.prototype.has(key)

Question 6

Removes the value by the key in a map object

Answer 6

Map.prototype.delete(key)

Question 7

Remove everything from the map object and return undefined

Answer 7

Map.prototype.clear()

Question 8

Returns the current element count in a map object

Answer 8

Map.prototype.size

Question 9

What does:
new Map()
do and/or return?

Answer 9

Create a new map

Question 10

What does:
Map.prototype.set()
do and/or return?

Answer 10

Store a value by the key in a map object
Map.prototype.set(key, value)
Returns the map itself (can chain)

Question 11

What does:
Map.prototype.get(key)
do and/or return?

Answer 11

Return the value by the key in a map object, undefined if key doesn’t exist in map.

Question 12

What does:
Map.prototype.has(key)
do and/or return?

Answer 12

Return true if the key exists in a map object, false otherwise.

Question 13

What does:
Map.prototype.delete(key)
do and/or return?

Answer 13

Removes the value by the key in a map object

Returns true or false based on whether the key existed or not.

Question 14

What does:
Map.prototype.clear()
do and/or return?

Answer 14

Remove everything from the map object

Returns undefined

Question 15

What does:
Map.prototype.size
do and/or return?

Answer 15

Returns the current element count in a map object

Question 16

Does:
map[key]
map.key
work with map objects?

Answer 16

Yes, and they will behave like regular objects in this way

Question 17

Returns iterables of keys, entries or values of a map objects

Answer 17

Map.prototype.keys()
Map.prototype.values()
Map.prototype.entries() (it’s used by default in for..of.
)

Question 18

What order do map objects iterate over keys, values and entries?

Answer 18

The order they were added.

Question 19

What does:
Map.prototype.forEach()
do and/or return?

Answer 19

.forEach does a function for each value in the map object

returns undefined

Question 20

Do a function for each item in the map object

2 ways

Answer 20

Map.prototype.forEach()

or use:
Map.prototype.keys()
Map.prototype.values()
Map.prototype.entries()
or the map itself (which is the same as .entries())
and a for of loop

Question 21

Create a new map objects with an array passed as an arugment to initiate some key/value pairs

Answer 21

let map = new Map([
  ['1',  'str1'],
  [1,    'num1'],
  [true, 'bool1']
]);

Question 22

Create a map object from a regular objects

Answer 22

Object.entries(obj) returns an array of the objects entries
Map objects can take an array at initialization to create some initial key/value pairs.

let obj = {
  name: "John",
  age: 30
};

let map = new Map(Object.entries(obj));

alert( map.get(‘name’) ); // John

Question 23

Create a regular object from a map (2 ways, similar. 1 better because shorter)

Answer 23

Object.fromEntries() can take an iterable object and create an object.

map.entries() can create an iterable object from the map object’s entries.(though map objects are already iterable)

So:
let someMaps= new Map();
someMaps.set('banana', 1);
someMaps.set('orange', 2);
someMaps.set('meat', 4);

let obj = Object.fromEntries(someMaps.entries());

But:
let obj = Object.fromEntries(map); // omit .entries()
Because Object.fromEntries expects an iterable object as the argument. Not necessarily an array. And the standard iteration for map returns same key/value pairs as map.entries(). So we get a plain object with same key/values as the map.

Question 24

What is a set

Answer 24

a special type of collection without keys. just sets of values. E

Question 25

a set can have multiples of the same value

t or f

Answer 25

false

Question 26

create a Set object

Answer 26

new Set(iterable)
copies any iterable objects values into the set

Question 27

What does:
Set.prototype.add()
do and/or return?

Answer 27

adds a value,

returns the set itself.

Question 28

What does:
Set.prototype.delete()
do and/or return?

Answer 28

removes the value,

returns true if value existed at the moment of the call, otherwise false.

Question 29

What does:
Set.prototype.has()
do and/or return?

Answer 29

returns true if the value exists in the set, otherwise false.

Question 30

What does:
Set.prototype.clear()
do and/or return?

Answer 30

removes everything from the set, returns undefined

Question 31

What does:
Set.prototype.size
do and/or return?

Answer 31

returns the elements count as a number

Question 32

What is a set object good for?

Answer 32

a way of ensuring multiple versions of the same value are not added in an efficient way.

Question 33

Iterate over a set object (2 ways)

Answer 33

for of
for (let value of set) alert(value);

Set.prototype.forEach()
set.forEach((value, valueAgain, set) => {
alert(value);
});

Question 34

Why does
Set.prototype.forEach()

have 2 arguments for the same value?
set.forEach((value, valueAgain, set) => {
alert(value);
});

Answer 34

To give it more compatibility with a map objects forEach method

Instead of having a different number of arguments. This makes it potentially easier to replace all maps with sets

Question 35

Return keys, values or entries as iterables from a set object

Answer 35

set. keys() – returns an iterable object for values,
set. values() – same as set.keys(), for compatibility with Map,
set. entries() – returns an iterable object for entries [value, value], exists for compatibility with Map.

Question 36

set objects are array like
t or f
set objects are iterable
t or f

Answer 36

True

True

Question 37

Create a function unique(arr) that should return an array with unique items of arr.

function unique(arr) {
  /* your code */
}

let values = [“Hare”, “Krishna”, “Hare”, “Krishna”,
“Krishna”, “Krishna”, “Hare”, “Hare”, “:-O”
];

alert( unique(values) ); // Hare, Krishna, :-O

Answer 37

alert( unique(values) );

Question 38

map objects are array like
t or f
map objects are iterable
t or f

Answer 38

True

True

Question 39

Create an array from a set or map

Answer 39

Array.from(set or map object))

Question 40

What happens to an Object key when you reassign a Map variable? 
let john = { name: "John" };

let myMap = new Map();
map.set(john, "...");

john = null;

Can you still get the john object?

Answer 40

Yes, you can get it using myMap.keys()

Question 41

What happens to an Object key when you reassign a weakMap variable? 
let john = { name: "John" };

let myMap = new Map();
map.set(john, "...");

john = null;

Can you still get the john object?

Answer 41

No

Everything gets wiped

Question 42

What are some differences between Map() and WeakMap() objects? (3 differences)

Answer 42

• WeakMap objects delete the object keys when the object used as the key is removed
• WeakMap has to have objects as keys. Map can use any value type as a key
• WeakMap does not take iteration methods like keys(), values, or entries.
  It only has:
  weakMap.get(key)
  weakMap.set(key, value)

weakMap.delete(key)
weakMap.has(key)

Question 43

Name all WeakMap methods

Why so few?

Answer 43

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

The JavaScript engine decides that. It may choose to perform the memory cleanup immediately or to wait and do the cleaning later when more deletions happen. So, technically, the current element count of a WeakMap is not known. The engine may have cleaned it up or not, or did it partially. For that reason, methods that access all keys/values are not supported.

Question 44

What is WeakMap good for?

2 uses

Answer 44

3. The main area of application for WeakMap is an additional data storage.

If we’re working with an object that “belongs” to another code, maybe even a third-party library, and would like to store some data associated with it, that should only exist while the object is alive – then WeakMap is exactly what’s needed.

We put the data to a WeakMap, using the object as the key, and when the object is garbage collected, that data will automatically disappear as well.

A Map() object would have to be manually cleaned once in a while.

2. Additionally it can be used for cach

Question 45

What is a WeakSet object?

Answer 45

A set that only allows objects that deletes them when the object is removed.

A regular Set() object will keep them stored?

Question 46

What methods are on WeakSet()

Answer 46

it supports add, has and delete,

but not size, keys() and no iterations.

Question 47

What is weakset used for?

Answer 47

Being “weak”, it also serves as additional storage. But not for arbitrary data, rather for “yes/no” facts. A membership in WeakSet may mean something about the object.

For instance, we can add users to WeakSet to keep track of those who visited our site: