Data types

Question 1

How do primitive methods work in general?

Answer 1

The JavaScript engine wraps a temporary object around the primitive that enables those methods.

Question 2

What is the “lazy” syntax for writing big numbers like 1,000,000 or small numbers like 0.0004?

Answer 2

Appending the letter “e” with the zeroes or negative zeroes count.
E.g. 1e6 === 1,000,000

Question 3

What are common numeral systems in JavaScript?

Answer 3

Besides decimal (10): hex (16), octal (8) and binary (2)

Question 4

How to write hexdecimal numbers?

Answer 4

Starting the number with 0x

Question 5

How to write octal numbers?

Answer 5

Starting the number with 0o

Question 6

How to write binary numbers?

Answer 6

Starting the number wie 0b

Question 7

How to transform a number from one numeral system into another?

Answer 7

parseInt(number, base)

Question 8

How to transform a string to a number in a specific numeral system?

Answer 8

number.toString(base)

Question 9

How to convert values like 12pt or 100px to numbers?

Answer 9

parseInt() or parseFloat()

Question 10

What are the limitations of parseInt() and parseFloat()?

Answer 10

The return the numbers from left to right to the first not-number character. Everything else get’s ignored.

Question 11

Which operations provide rounding of numbers?

Answer 11

Math.floor() rounds down to nearest integer; Math.ceil() rounds up to nearest integer; Math.round() rounds to nearest integer; Math.trunc() removes everything after the decimal point

toFixed(n) rounds the number to n digits after the decimal point (but returns a string)

Question 12

How to check for NaN?

Answer 12

isNaN() - NaN is special, is does not equal to anything so comparison doesn’t work

Question 13

What does isFinite(value) do?

Answer 13

Converts value to number an returns true if number is not Infinity, -Infinity or NaN

Question 14

Why do you pay attention when working with fractions?

Answer 14

Fractions are imprecise, that’s why 0.1 + 0.2 == 0.3 // false

Question 15

Why is this while loop infinite?

let i = 0;
while ( != 10) {
   i += 0.2;
}

Answer 15

i would never be exactly 10 because of imprecise fractions in JavaScript

Question 16

How to wrap a string?

Answer 16

With single quotes ‘ ‘, double quotes “ “ or backticks ` `

Question 17

What is the difference between single quotes, double quotes and backticks?

Answer 17

Single and double quotes are the same. Backticks are a new ES6 feature and allow multiple lines and embedded expressions with ${…}

Question 18

How are strings in JavaScript encoded?

Answer 18

UTF-16

Question 19

How can we use special characters in strings?

Answer 19

By escaping them with \ or using unicode characters with \u…

Question 20

How to access a single character in a string?

Answer 20

string[]

Question 21

How to get a substring?

Answer 21

.slice(start [, end]) or .substring(start [,end])

slice includes start but not end
substring only string between start and end

Question 22

How to lowercase or uppercase a string?

Answer 22

.toLowerCase and .toUpperCase

Question 23

How to find substrings?

Answer 23

.indexOf(substr, pos) -> returns position where substr starts

.includes(substr) -> returns true/false
.startsWith(substr) -> returns true/false
.endsWith(substr) -> returns true/false

Question 24

How does string comparison work?

Answer 24

Letter by letter which is in the alphabetical order (or better which numeric code in UTF-16 is smaller)

Capital letters are smaller than lowercase letters

Question 25

How to compare strings correct?

Answer 25

For different languages string comparison works with .localeCompare()

Question 26

Explain str.trim()

Answer 26

Trims whitespace of string from beginning and end

Question 27

Explain str.repeat(n)

Answer 27

Repeats the strings n times

Question 28

How to declare an array?

Answer 28

let arr = [item1, item2, ...] ``` or rare syntax: let arr = new Array(item1, item2, ...) ```

Question 29

What is special about new Array(2)?

Answer 29

It creates and empty array with length equals 2

Question 30

Explain the length property?

Answer 30

Special property of arrays which give the length or, better the last numeric index plus one

Question 31

What happens if we shorten the length property?

Answer 31

We truncate the array

Question 32

How to add or remove items from an array?

Answer 32

push(...items) - add to the end pop() - remove from the end and return removed one shift(...items) - add to the beginning unshift() - remove from the beginning and return removed one

Question 33

How to loop over an array?

Answer 33

for loop -> classic and fastests for of -> modern syntax for in -> possible but for objects optimized

Question 34

Explain arr.splice()

Answer 34

Syntax: arr.splice(index[, deleteCount, elem1, ... elemN]) Starts at index, deletes deleteCount elements and adds elem1... elemN It is possible to only remove or only add.

Question 35

Explain arr.slice()

Answer 35

Syntax: arr.slice(start, end) Returns a new strings, beginning at start and ending (not including) at end.

Question 36

Explain arr.concat()

Answer 36

Syntax: arr.concat(arg1, arg2, ...) Joins multiple Arrays/items. Turns joined arrays in items of the new array.

Question 37

Explain arr.forEach()

Answer 37

Syntax: arr.forEach(callback) Iterates over array and calls callback on every element.

Question 38

Explain arr.indexOf(), arr.lastIndexOf() and arr.includes()

Answer 38

Searches for item (starting from position) and returns the position or true/false (includes)

Question 39

Explain arr.find() and arr.findIndex()

Answer 39

Syntax: arr.find(callback(item, index, array)) Callback is called for each item, if it returns true the item is returned, else undefined. findIndex() returns the index value.

Question 40

Explain arr.filter()

Answer 40

Syntax: arr.filter(callback(item, index, array)) arr.find() returns the first match, filter() returns all matches in a new array

Question 41

Explain arr.map()

Answer 41

Syntax: arr.map(function(item, index, array) {}) Used to transform items in an array. Calls function on each element and returns array of results.

Question 42

Explain arr.sort()

Answer 42

Returns the sorted array.

Question 43

Explain arr.reverse()

Answer 43

arr.reverse() reverses the ordering of elements in an array

Question 44

Explain str.split() and arr.join()

Answer 44

str. split(delimiter) splits a string after each delimiter and turns it into an array. arr. join(delimiter) turn a array into an string with delimiter between each element