Javascript (Hack Reactor)

Question 1

What does ‘NaN’ mean?

Answer 1

Not A Number
i.e. “I think you meant to have a number here, but I cannot produce a number with what you gave me, so have NaN instead :)

Question 2

Data types (8)

Answer 2

Scalar:
(1) Number
(2) BigInt
(3) String
(4) Boolean (logical type)
(5) null
(6) undefined

(7) Objects and (8) Symbols

Question 3

How to format a string to be replaced with variables?

Answer 3

Use backticks
` random text ${variable} here `

Question 4

What is the difference in formatting a string with variable replacements in JS and formatting a string in Python?

Answer 4

In Python we format strings using f strings e.g.
f’random text {variable} here’

In JS we use backticks and preface variable replacement with $
` random text ${variable} here `

Question 5

Method to find the length of a string?

Answer 5

string.length

Question 6

How to format if else statement?

Answer 6

if (cond) {
// code to execute
} else {
// code to execute
}

Question 7

How to format function?

Answer 7

function checkAge(age) {
# code
}

Question 8

How to specify a line as a comment?

Answer 8

Preface with // (forward slashes)

Question 9

How to delete the key and value of an object?

Answer 9

delete obj[key]

Question 10

How to access the value of an object?

Answer 10

bracket notation
obj[key]

Question 11

Does NULL == undefined?

Answer 11

No, these are different data types

Question 12

In what two scenarios would ‘undefined’ be returned?

Answer 12

• when we do not specify what should be returned in our function
• variable declared at but not assigned a value

Question 13

Booleans in JS vs booleans in Python

Answer 13

lowercase true or false

Python, the booleans are capitalised True or False

Question 14

Which equality operator should we use to err on the side of caution?

Answer 14

triple equals

Question 15

How to express these operators:
and
or

Answer 15

symbols
&&
||