Containers

Question 1

Create array from iterable (e.g. Set, Map)

Answer 1

Array.from(new Set([“foo”, “bar”, “baz”, “foo”]));

Question 2

Check if arr is an array

Answer 2

Array.isArray(arr)

Question 3

Truncate array to only 3 elements

Answer 3

a.length = 3

Question 4

Add/Remove single element from front/back of an array arr (all methods)

Answer 4

arr.push(5);
arr.pop();
arr.unshift("one");
arr.shift();

Question 5

Add multiple elements to the front / back of an array arr

Answer 5

arr.push(1, 2, 3);
arr.unshift(1, 2, 3);

Question 6

Add one array to another (all methods)

Answer 6

["1", "2", "3"].concat("a", "b", "c");

a.concat(b)

[...[1, 3, 2], ...[4, 5, 7]]

// Note:
// [1, 2, 3] + [3, 4, 5]
// will add string representations resulting in '1,2,33,4,5

Question 7

Join elements of an array with -

Answer 7

["1", "2", "3"].join("-");

Question 8

All array search methods

Answer 8

arr.indexOf("b", 3);
arr.lastIndexOf("b", 3);
arr.find((e) => e > 10); // returns THAT ELEMENT
arr.findIndex((e) => e > 10); // returns index
arr.findLast((e) => e > 10);
arr.findLastIndex((e) => e > 10);

Question 9

Check if 2 is in array arr

Answer 9

arr.includes(2);

Question 10

Reverse and sort an array (in place and with copy)

Answer 10

arr.reverse();  // in place
cont reversedArr = arr.toReversed();
const sorted = arr.toSorted();
arr.sort(); // in place

Question 11

Create array of 10 elements all with value 4

Answer 11

const tenFours = Array(10).fill(4)

Question 12

All iterative array methods, args where necessary

And what they do

Answer 12

arr.forEach
arr.map
arr.flatMap
arr.filter
arr.every
arr.some
reduce(
  (accumulator, currentValue) => accumulator + currentValue,
  initialValue,
);
arr.reduceRight

Question 13

Get subarray

Answer 13

months.slice(1, 3); // Indexes [1, 3)

Question 14

What is splice

Answer 14

// Splice (splice = splot)
// >> splice(start, deleteCount, item1, item2, /* …, */ itemN)
// Remove at index [1] 2 elements, replace with 'a' and 'b', returns removed elements
const removed = months.splice(1, 2, "a", "b"); // [1, "a", "b", 4, 5]

Question 15

Convert ArrayBuffer to regular array of 32-bit integers

Answer 15

let buffer = new ArrayBuffer(16);
let int32View = new Int32Array(buffer);
const normalArray = Array.from(int32View);

Question 16

• Assign 1’st element of array to a
• 2’nd element of an array to b
• Remaining elements to everythingElse (as an array)

In single line

Answer 16

const [a, b, …everythingElse] = [0, 1, 1, 2, 3, 5, 8];

Question 17

Methods on Map
* Add and remove elements
* Read elements
* Check if element in a map
* Remove all elements
* Get length

Answer 17

sayings.set("dog", "woof");
sayings.delete("dog");
sayings.get("dog"); 
sayings.has("bird");
sayings.clear();
sayings.size;

Question 18

Methods on Set
* Add and remove elements
* Check if element in a set
* Remove all elements
* Get length

Answer 18

mySet.add(1);
mySet.delete(1);
mySet.has(1);
mySet.clear();
mySet.size;

Question 19

Iterate all Map entries

Answer 19

for (let [key, value] of sayings) {
  console.log(key + " goes " + value);
}

Question 20

Iterate all Set entries

Answer 20

for (let item of mySet) console.log(item);

Question 21

How to use async callback in map

Answer 21

const itemsPromises = [1, 2, 3].map(async (i) => i.toString())
await Promise.all(itemsPromises);

Question 22

Flatten container to 1D
(Something like [0, 1, 2, [3, 4]])

Answer 22

Use flat(depth)

[0, 1, 2, [3, 4]].flat()