Typescript Basics

Question 1

Loop through an object’s keys and values

Answer 1

for (const [key, value] of Object.entries(animals)) {
console.log(${key}: ${value});
}

Question 2

Check if a key is in an object

Answer 2

if (key in myObject) { … }

Question 3

*Loop through values in an array

Answer 3

for (const num of nums) { … }

Question 4

Sort an array in ascending order

Answer 4

array.sort((a, b) => a - b)

Question 5

Define a tree structure (eg BST)

Answer 5

class TreeNode {
value: number;
left: TreeNode | null;
right: TreeNode | null;

constructor(value: number) {
this.value = value;
this.left = null;
this.right = null;
}
}

const node = new TreeNode(10)

Question 6

Write a switch statement

Answer 6

const expr = ‘Papayas’;
switch (expr) {
case ‘Oranges’:
console.log(‘Oranges are $0.59 a pound.’);
break;
case ‘Mangoes’:
case ‘Papayas’:
console.log(‘Mangoes and papayas are $2.79 a pound.’);
break;
default:
console.log(Sorry, we are out of ${expr}.);
}

Question 7

Round decimal value down to nearest integer

Answer 7

Math.floor(n)

Question 8

Round decimal value up to nearest integer

Answer 8

Math.ceil(n)

Question 9

Round decimal value towards zero

Answer 9

Math.trunc(n)

Question 10

Remove first element from array

Answer 10

const value = array.shift()

Question 11

Get the max value in an array

Answer 11

Math.max(…array)

Question 12

Create an empty array of n items

Answer 12

new Array(n).fill(null)

Question 13

*Loop through indexes and values of array

Answer 13

for (const [index, value] of myArray.entries()) { … }

Question 14

Remove key from object

Answer 14

delete myObj[key]

Question 15

Get array of keys from object

Answer 15

Object.keys(myObj)

Question 16

Convert number to string

Answer 16

n.toString()
OR
String(n)

Question 17

Convert string to number

Answer 17

Number(x)

Question 18

Create a class called Range that takes from and to as number arguments and includes a method called toString that prints both. Then instantiate an instance of the class and call toString

Answer 18

class Range {
constructor(from, to) {
this.from = from;
this.to = to;
}
toString() {
return ${this.from}...${this.to};
}
}

const range = new Range(1, 4);
range.toString();

Question 19

Create an async sleep function that resolves after n milisecondes

Answer 19

const sleep = async (n: number): Promise<void> => {
return new Promise(resolve => setTimeout(resolve, n));
}</void>

Question 20

Create a timeout and cancel a timeout

Answer 20

const timeout = setTimeout(() => {…}, 10);
clearTimeout(timeout);

Question 21

Given an array, return the 1st to 3rd (inclusive) elements as an array

Answer 21

array.slice(0, 3)

Question 22

*Implement and use a Map (and iterate through keys and values of map)

Answer 22

const myMap = new Map();
myMap.set(‘a’, 1)
myMap.get(‘a’)
myMap.has(‘a’)
myMap.size // 1
myMap.delete(‘a’)

for (const [key, value] of myMap) {…}

Question 23

*Implement and use a Set (and iterate through values of set)

Answer 23

const mySet = new Set(); // use new Set(array) to convert array to set
mySet.add(‘a’);
mySet.has(‘a’); // true
mySet.size; // 1
mySet.delete(‘a’);

for (const item of mySet) {…}
const myArray = […mySet]

Question 24

Use reduce method to sum numbers in an array

Answer 24

[1, 2, 3].reduce((acc, curr) => acc + curr, 0)

Question 25

Set the largest (smallest) number possible to the variable max (min)

Answer 25

const max = Infinity;
const min = -Infinity;

Question 26

Determine if all elements in an array are below 4

Answer 26

[1, 2, 3].every((value) => value < 4)

Question 27

Given an array, remove all elements that are less than 0

Answer 27

[-1, 2, 3].filter((value) => value >= 0);

Question 28

Find the first index in an array for a specific value

Answer 28

[1, 2, 3].findIndex((el) => el === 2);
OR arr.indexOf(2)

Question 29

Find the last index in an array of a specific value

Answer 29

[1, 2, 2, 3].findLastIndex((el) => el === 2);
OR arr.lastIndexOf(2)

Question 30

Given the array [1, 2, [3, [4]]], generate the array [1, 2, 3, 4]

Answer 30

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

Question 31

*Determine if an array contains a value

Answer 31

[1, 2, 3, 4].includes(3);

Question 32

*Determine if a variable is an array type

Answer 32

Array.isArray(myVar);

Question 33

How do you run a function every n milliseconds?

Answer 33

const timerId = setInterval(() => myFunction, n);
clearInterval(timerId);

Question 34

Create a new date

Answer 34

const date = new Date(year, month, day)
// note, month is zero-indexed

Question 35

How do you check if a variable is an object?

Answer 35

typeof x === ‘object’ && x !== null && !Array.isArray(x)