## Number

• JavaScript has a built-in type called `Number`.
• It represents decimal numbers, aka floating-point decimals.
• JavaScript does not have a special type for integers, unlike most other languages.

MDN Docs for Number

## Numbers

The following operations work on numbers:

operand name example =
+ addition 3 + 2 5
- subtraction 3 - 2 1
* multiplication 3 * 2 6
/ division 3 / 2 1.5
% modulus ("remainder") 3 % 2 1
** exponentiation ("power of") 3 ** 2 9

## LAB: Playing With Numbers

Answer the following questions using `node`:

• How many seconds are in an hour?
• How many minutes are in a week?
• How many seconds old are you?
• How many years old is someone who is 1 billion seconds old?

## Order of operations

Q: What is 1 plus 2 times 3?

## Order of operations

Q: What is 1 plus 2 times 3?

A: It depends!

``````(1 + 2) * 3 == 9
1 + (2 * 3) == 7
``````

## Parentheses Are Free

When in doubt, use parentheses!

(or, when there's any doubt, make your assumptions explicit)

## LAB: one nation, divisible

One number is divisible by another if you divide them and the remainder is 0.

Write code in `node` to figure out if...

• 100 is divisible by 5
• 100 is divisible by 8
• 330 is divisible by 11
• 337 is divisible by 7

## Strings vs. Numbers

Hmmm....

``````1 + 2
"1" + "2"
"1 + 2"
``````

## Strings plus Numbers

Hmmm again...

``````"1" + 2
``````

Even though Strings and Numbers are different TYPES, JavaScript converts one to the other.

But when two types meet, which one wins?

## Type Conversion

The clearest answer is that you, the programmer, explicitly declare which type you want to win.

## Converting a Number to a String

``````(12).toString() // "12"
``````

## Converting a String to a Number

The easiest and cleanest is `unary +`:

expression value
+"12" 12
+"012" 12
+"0.2" 0.2
+"cheese" NaN
+"0" 0
+"" 0
+" " 0

(Other ways can give bizarre results if the string contains letters or begins with a 0.)

## Type Coercion

If you don't explicitly do type conversion then JavaScript will "helpfully" convert the types for you.

This is called type coercion and just like with people, coercion is stressful and often backfires.

For instance, if you ask the user their age, and read it from a web form or keyboard input, it will be in a string. If you forget to explicitly convert that to a number before using it in a calculation, the results can be unexpected...

``````  > "30" - 1              // "30" is coerced into a number
29
> "30" + 1              // 1 is coerced into a string
'301'
``````

(You Don't Know JS has way more information than you wanted to know on this subject.)

## Randomize it!

Math.random is your way to make things unpredictable!

Calling `Math.random()` returns a random decimal number between 0 and 1.

If you want a random integer you can multiply that decimal by a range, like this:

``````function randomInteger(min, max) {
let range = max - min + 1;
return min + Math.floor(Math.random() * range);
}

console.log(randomInteger(1, 3));
``````

## Math is hard

There are many types of numbers!

Each is useful in different situations.

In elementary school, we learned

• natural numbers (aka counting numbers)
• integer numbers (naturals, negatives, and zero)
• rational numbers (aka fractions or ratios)

In high school, we learned irrational, complex, and imaginary numbers.

JavaScript numbers are different from all of those!

## Floating-Point Numbers

JavaScript numbers...

• use decimal notation
• are stored in binary
• have a fixed bit-length

Some rational numbers cannot be represented in floating-point, which means that simple arithmetic may give unexpected results.

For instance, you can't go higher than about 9 quadrillion without glitching...

``````> 2**53 == 2**53+1
true
``````

(This problem is not unique to JavaScript, but the fact that all numbers in JS are floating-point means that beginners can't avoid it.)

see Wikipedia on IEEE 754 double aka Double-precision floating-point format or binary64

## Arithmetic

Try this in node:

``````1 + 2
3 - 4
5 * 6
7 / 8
2 / 3
7 / 9
``````

Wait a second...

``````> 2/3
0.6666666666666666
> 7/9
0.7777777777777778
``````

Why did it round up the last digit for `7/9` but not for `2/3`?

## Floating Point Arithmetic is Bizarre

``````> 7/9
0.7777777777777778

> 0.5 - 0.4 - 0.1
-2.7755575615628914e-17

> (0.8 - 0.7 - 0.1)/(0.5 - 0.4 - 0.1)
-3

> 2**53 == 2**53+1
true

> 2**10000
Infinity
``````