Numbers Knowledge

Question 1

‘8’ == 8; ‘8’ === 8;

Answer 1

true false

Question 2

0.1 + 0.2

Answer 2

0.30000000000000004

Question 3

Return the biggest number

What happens with bigger numbers?

Answer 3

Number.MAX_VALUE;

// 1.7976931348623157e+308

They are returned as Infinity

Question 4

‘5’ + 1

‘1’ + 5

Answer 4

‘51’

‘15’

Question 5

‘51’ > 6

6 < ‘51’

Answer 5

true

true

Question 6

‘5’ * 1

‘5’ / ‘1’

Answer 6

5

5

Question 7

‘12’ < ‘9’

Answer 7

true it only converts the first character of each if both sides are strings

Question 8

Given an array of integers, find the one that appears an odd number of times.

There will always be only one integer that appears an odd number of times.

Answer 8

use an XOR gate ^
const a = 5; // 00000000000000000000000000000101
const b = 3; // 00000000000000000000000000000011

(a ^ b); // 00000000000000000000000000000110

const findOdd = (xs) => xs.reduce((a, b) => a ^ b);

Question 9

Get infinity (2 ways)

Answer 9

Infinity

1/0

Question 10

Evaluate Number(x) when x equals:

undefined
null
true and false
‘ 5 ‘

’ ‘

’ 2 hello ‘

Answer 10

NaN

0

1 and 0

5

0

NaN.

Question 11

Evaluate:

‘2’ + 1

2 + ‘1’

Answer 11

‘21’

‘21’

Question 12

Evlaluate:

2 + 2 + ‘1’

‘1’ + 2 + 2

Answer 12

41

122

Question 13

Evaluate:

“6” / “2”

‘6’ - 2

‘a’ - 2

Answer 13

3

4

NaN

(+ is the only operator that will implicitly convert to strings. The others will only implicitly convert to numbers)

Question 14

Evaluate the following with the + unary operator:

+2

+(-2)

+null

+undefined

+“true”

+true

Answer 14

It does the same thing as Number() essentially

2

-2

0

NaN

NaN

1

Question 15

Evaluate a, b and c:

let a, b, c;

a = b = c = 2 + 2;

Answer 15

4

4

4

(but don’t do it)

Question 16

Evaluate a and b:

let counter = 1;

let a = ++ counter

let b = counter++

Answer 16

a = 2 (add 1 to counter, then assign it to a)

b= 1 (assign counter to b, then add 1 to counter)

Question 17

Evaluate

let counter = 0;

console. log(2 * counter++);
console. log(2 * ++counter);

Answer 17

0

4

Question 18

For readability, instead of:

let counter = 1;

alert( 2 * counter++ );

what should you do?

Answer 18

let counter = 1;
alert( 2 * counter );
counter++;

Question 19

What is returned from:

let a = (1 + 2, 3 + 4);

a;

Answer 19

// 7 (the result of 3 + 4)

Each of them is evaluated but only the result of the last one is returned.

Question 20

What is returned from:

let a = 1 + 2, 3 + 4;

and what is returned from:

a;

Answer 20

//syntax error

Question 21

What is returned from:

true + false

Answer 21

1

+ converts them to numbers

1 + 0

Question 22

What is returned from:

” -9 “ - 5

Answer 22

-14

it first removes blank spaces from the beginning and end

Question 23

This returns 12, make it return 3

let a = prompt("First number?", 1);
let b = prompt("Second number?", 2);

alert(a + b); // 12

Answer 23

let a = +prompt(“First number?”, 1);
let b = +prompt(“Second number?”, 2);

alert(a + b); // 3

OR

let a = prompt("First number?", 1);
let b = prompt("Second number?", 2);

alert(+a + +b); // 3

Question 24

Boolean(0)

Boolean(‘0’)

0 == ‘0’

Answer 24

False

True

True

Question 25

‘2’ > ‘12’

Answer 25

true

(both are strings so 2 > 1)

Question 26

‘12’ < 2

Answer 26

false

(‘12’ is converted to a number when compared to a number)

Question 27

alert( null > 0 );
alert( null == 0 );
alert( null >= 0 );

Answer 27

// (1) false

// (2) false (with equality, null is not converted to a number)

// (3) true (with comparisons, null is converted to a number)

Question 28

What is returned from:

123.toString()

?

Answer 28

Syntax error.

it works with (123).toString()

or

let aNumber = 123

aNumber.toString()

Why? [incomplete]

Question 29

What is an integer property?

Answer 29

a string that can be converted to a number and back to a string without changing

alert( String(Math.trunc(Number(“49”))) ); // “49”, same, integer property

alert( String(Math.trunc(Number(“+49”))) ); // “49”, not same “+49” ⇒ not integer property

alert( String(Math.trunc(Number(“1.2”))) ); // “1”, not same “1.2” ⇒ not integer property

Question 30

What is the value of x in the following:

let x = 123_123

let x = 123__123

Answer 30

123123
error

Question 31

Write 1123000000 in exponential form

write .0123 in exponential form

Answer 31

1.234e9
1.23e-2

Question 32

Convert numbers from binary

Convert numbers from octal

Answer 32

let a = 0b11111111; // binary form of 255

let b = 0o377; // octal form of 255

alert( a == b ); // true, the same number 255 at both sides

Question 33

Shorthand for hexadecinal numbers

Answer 33

0x

console.log(0xff);

Question 34

Convert numbers to a specific base numbering system as a string

Answer 34

(15).toString(16)

Question 35

Write a function to round number to specific number of decimal places using Math.round()

Answer 35

Math.round() just rounds to the nearest integer. So:

num = 1.23456

Math.round(num * 100) / 100 ; // 1.23456 -> 123.456 -> 123 -> 1.23

A function that would do it might look like:

function roundToFloat(inputNumber, numberOfDecimalPlaces) {

return Math.round(inputNumber * (10**(numberOfDecimalPlaces))) / 10**(numberOfDecimalPlaces);

}

Question 36

What is returned from:

1e500

Answer 36

infinity

Question 37

What does this return (16 nines):

9999999999999999

Answer 37

10000000000000000
Javascript doesn’t have integers, only 64-bit floats - and you’ve ran out of floating-point precision.

Question 38

+‘Infinity’ == Infinity

Answer 38

true

Question 39

What is returned from 1e(variable)

Answer 39

an error

to take the power of a variable, use **