Basic Concepts

Question 1

JavaScript is defined as…

Answer 1

A lightweight, multi-paradigm, single-threaded, dynamic scripting language that is found in all modern web technologies

Question 2

The main 8 types of the JavaScript language

Answer 2

1) Number
2) BigInt
3) String
4) Boolean
5) Object
6) Null
7) Undefined
8) Symbol

Question 3

How is the Number type implemented

Answer 3

As a double-precision 64-bit binary format IEEE 754 value

Question 4

What are some subclasses of Objects available to you in JavaScript

Answer 4

• Function
• Array
• Date
• RegExp
• Math

Question 5

parseInt()

Answer 5

this will parse a string into an integer value, where the first parameter is the string, and the second is the base you want to parse out.

Question 6

NaN

Answer 6

This means ‘Not a Number’, and is the result of trying to use a non-number value in a number context.
i.e. parseInt(‘hello’, 10);

use Number.isNaN(value) to check for NaNness

Question 7

NaN is ‘toxic’

Answer 7

Any mathematical operation with a NaN, will result in NaN

Question 8

parseFloat()

Answer 8

Similar to parseInt, but only uses base 10 and will output the floating point representation of the value parsed.

Question 9

Falsy Values

Answer 9

False, 0, “”, NaN, null, undefined

Question 10

Truthy Values

Answer 10

Anything that is not Falsy

Question 11

let vs. const. vs. var

Answer 11

let: declares a mutable variable in block scope
const: declares an immutable variable in block scope
var: declares a mutable variable with global scope (DONT USE)

Question 12

for…in vs. for…of

Answer 12

for in is used to iterate over enumerable values. It is good for checking the existence of keys in a map/object. does not guarantee order of enumeration.
for of is useful for iteration of iterable values, so arrays, strings, etc. more often than not, this will be the preferred one.

Question 13

x && x.name

Answer 13

as long as x exists, return x.name

Question 14

x.name || x.getName()

Answer 14

if x.name doesn’t exist, get it

Question 15

A variable can start with

Answer 15

Any letter along with $ and _

Question 16

??

Answer 16

Nullish Coalescence Operator
a ?? b
if ‘a’ is null/undefined then ‘b’, otherwise ‘a’

Question 17

== vs. ===

Answer 17

Equality vs. Strict Equality
== : this will attempt to convert the types of the value on each side of the equals sign into the same type before comparing.
===: this will do no such comparison and will strictly compare the two values for equality

Question 18

comma operator

Answer 18

evaluates everything in the expression, from left to right, and returns the final expression evaluated. (the rightmost value after the final comma)

Question 19

function (x = 1, x = 2) {}

Answer 19

Functions can be given default parameters, this allows for default values to be used in the event that none are passed in

Question 20

Transpilers

Answer 20

Special piece of software that translates one source code into another. Main difference between this and a compiler, is there is no speed/performance improvement intended. It would be like translating python into ruby.

ex. of transpilers
Babel

Question 21

Polyfills

Answer 21

Basically a pollyfill is a function that exists because the actual function you are using is not supported yet in some browsers.

This is a consequence of the language being updated without full support for its updates. So certain functions don’t actually have any meaning in older browsers (ex. Math.trunc()) and have to be substituted with a “fill”

Question 22

?.

Answer 22

Optional Chaining
Allows you to safely access nested values of an object, even if they, or the object, or a subcall doesn’t exist

it will return undefined in the case where something along the chain does not exist, rather than throwing an error

can also use ?.() for optional method calls and ?.[] for optional array access

Question 23

Symbols

Answer 23

Guaranteed to be Unique

They are a new addition to the language

Question 24

“use strict”

Answer 24

Places the execution environment into strict mode.
Makes it easier to write secure JavaScript
This essentially does 3 things
1) Eliminates some JS silent errors by changing them to throw errors
2) Fixes mistakes that make it difficult for JS engines to make optimizations
3) Prohibits some syntax from being used

Question 25

toFixed(number)

Answer 25

A number method that rounds to a fixed decimal place and returns it as a string

Question 26

Number, String, Boolean wrappers

Answer 26

These are wrappers that will covert the value passed in into the type of the wrapper. Using new will create an object, but without new, it is just a cast essentially.

Question 27

1_000_000

Answer 27

The underscore is a valid separator for numbers and will be parsed as if they were not there

Question 28

Math. floor, ceil, round, trunc

Answer 28

floor: rounds down
ceil: rounds up
round: rounds to nearest integer
trunc: removes anything after the decimal (no rounding)

Question 29

isFinite(value), isNaN(value)

Answer 29

Infinity and Nan checks

Question 30

Object.is(a,b)

Answer 30

This is another way to use ===