In modern JavaScript, there are two types of numbers:
Regular numbers in JavaScript are stored in 64-bit format IEEE-754, also known as "double precision floating point numbers". These are numbers that we're using most of the time, and we'll talk about them in this chapter.
BigInt numbers, to represent integers of arbitrary length. They are sometimes needed, because a regular number can't exceed 253 or be less than -253. As bigints are used in few special areas, we devote them a special chapter info:bigint.
So here we'll talk about regular numbers. Let's expand our knowledge of them.
Imagine we need to write 1 billion. The obvious way is:
let billion = 1000000000;
We also can use underscore _ as the separator:
let billion = 1_000_000_000;
Here the underscore _ plays the role of the "syntactic sugar", it makes the number more readable. The JavaScript engine simply ignores _ between digits, so it's exactly the same one billion as above.
In real life though, we try to avoid writing long sequences of zeroes. We're too lazy for that. We'll try to write something like "1bn" for a billion or "7.3bn" for 7 billion 300 million. The same is true for most large numbers.
In JavaScript, we can shorten a number by appending the letter "e" to it and specifying the zeroes count:
let billion = 1e9; // 1 billion, literally: 1 and 9 zeroes
alert( 7.3e9 ); // 7.3 billions (same as 7300000000 or 7_300_000_000)
In other words, e multiplies the number by 1 with the given zeroes count.
1e3 === 1 * 1000; // e3 means *1000
1.23e6 === 1.23 * 1000000; // e6 means *1000000
Now let's write something very small. Say, 1 microsecond (one millionth of a second):
let ms = 0.000001;
Just like before, using "e" can help. If we'd like to avoid writing the zeroes explicitly, we could say the same as:
let ms = 1e-6; // six zeroes to the left from 1
If we count the zeroes in 0.000001, there are 6 of them. So naturally it's 1e-6.
In other words, a negative number after "e" means a division by 1 with the given number of zeroes:
// -3 divides by 1 with 3 zeroes
1e-3 === 1 / 1000; // 0.001
// -6 divides by 1 with 6 zeroes
1.23e-6 === 1.23 / 1000000; // 0.00000123
Hexadecimal numbers are widely used in JavaScript to represent colors, encode characters, and for many other things. So naturally, there exists a shorter way to write them: 0x and then the number.
For instance:
alert( 0xff ); // 255
alert( 0xFF ); // 255 (the same, case doesn't matter)
Binary and octal numeral systems are rarely used, but also supported using the 0b and 0o prefixes:
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
There are only 3 numeral systems with such support. For other numeral systems, we should use the function parseInt (which we will see later in this chapter).
The method num.toString(base) returns a string representation of num in the numeral system with the given base.
For example:
let num = 255;
alert( num.toString(16) ); // ff
alert( num.toString(2) ); // 11111111
The base can vary from 2 to 36. By default it's 10.
Common use cases for this are:
base=16 is used for hex colors, character encodings etc, digits can be 0..9 or A..F.
base=2 is mostly for debugging bitwise operations, digits can be 0 or 1.
base=36 is the maximum, digits can be 0..9 or A..Z. The whole latin alphabet is used to represent a number. A funny, but useful case for 36 is when we need to turn a long numeric identifier into something shorter, for example to make a short url. Can simply represent it in the numeral system with base 36:
alert( 123456..toString(36) ); // 2n9c
Please note that two dots in `123456..toString(36)` is not a typo. If we want to call a method directly on a number, like `toString` in the example above, then we need to place two dots `..` after it.
If we placed a single dot: `123456.toString(36)`, then there would be an error, because JavaScript syntax implies the decimal part after the first dot. And if we place one more dot, then JavaScript knows that the decimal part is empty and now goes the method.
Also could write `(123456).toString(36)`.
One of the most used operations when working with numbers is rounding.
There are several built-in functions for rounding:
Math.floor
: Rounds down: 3.1 becomes 3, and -1.1 becomes -2.
Math.ceil
: Rounds up: 3.1 becomes 4, and -1.1 becomes -1.
Math.round
: Rounds to the nearest integer: 3.1 becomes 3, 3.6 becomes 4, the middle case: 3.5 rounds up to 4 too.
Math.trunc (not supported by Internet Explorer)
: Removes anything after the decimal point without rounding: 3.1 becomes 3, -1.1 becomes -1.
Here's the table to summarize the differences between them:
Math.floor |
Math.ceil |
Math.round |
Math.trunc |
|
|---|---|---|---|---|
3.1 |
3 |
4 |
3 |
3 |
3.6 |
3 |
4 |
4 |
3 |
-1.1 |
-2 |
-1 |
-1 |
-1 |
-1.6 |
-2 |
-1 |
-2 |
-1 |
These functions cover all of the possible ways to deal with the decimal part of a number. But what if we'd like to round the number to n-th digit after the decimal?
For instance, we have 1.2345 and want to round it to 2 digits, getting only 1.23.
There are two ways to do so:
Multiply-and-divide.
For example, to round the number to the 2nd digit after the decimal, we can multiply the number by 100 (or a bigger power of 10), call the rounding function and then divide it back.
let num = 1.23456;
alert( Math.round(num * 100) / 100 ); // 1.23456 -> 123.456 -> 123 -> 1.23
The method toFixed(n) rounds the number to n digits after the point and returns a string representation of the result.
let num = 12.34;
alert( num.toFixed(1) ); // "12.3"
This rounds up or down to the nearest value, similar to Math.round:
let num = 12.36;
alert( num.toFixed(1) ); // "12.4"
Please note that result of toFixed is a string. If the decimal part is shorter than required, zeroes are appended to the end:
let num = 12.34;
alert( num.toFixed(5) ); // "12.34000", added zeroes to make exactly 5 digits
We can convert it to a number using the unary plus or a Number() call: +num.toFixed(5).
Internally, a number is represented in 64-bit format IEEE-754, so there are exactly 64 bits to store a number: 52 of them are used to store the digits, 11 of them store the position of the decimal point (they are zero for integer numbers), and 1 bit is for the sign.
If a number is too big, it would overflow the 64-bit storage, potentially giving an infinity:
alert( 1e500 ); // Infinity
What may be a little less obvious, but happens quite often, is the loss of precision.
Consider this (falsy!) test:
alert( 0.1 + 0.2 == 0.3 ); // *!*false*/!*
That's right, if we check whether the sum of 0.1 and 0.2 is 0.3, we get false.
Strange! What is it then if not 0.3?
alert( 0.1 + 0.2 ); // 0.30000000000000004
Ouch! There are more consequences than an incorrect comparison here. Imagine you're making an e-shopping site and the visitor puts $0.10 and $0.20 goods into their cart. The order total will be $0.30000000000000004. That would surprise anyone.
But why does this happen?
A number is stored in memory in its binary form, a sequence of bits - ones and zeroes. But fractions like 0.1, 0.2 that look simple in the decimal numeric system are actually unending fractions in their binary form.
In other words, what is 0.1? It is one divided by ten 1/10, one-tenth. In decimal numeral system such numbers are easily representable. Compare it to one-third: 1/3. It becomes an endless fraction 0.33333(3).
So, division by powers 10 is guaranteed to work well in the decimal system, but division by 3 is not. For the same reason, in the binary numeral system, the division by powers of 2 is guaranteed to work, but 1/10 becomes an endless binary fraction.
There's just no way to store exactly 0.1 or exactly 0.2 using the binary system, just like there is no way to store one-third as a decimal fraction.
The numeric format IEEE-754 solves this by rounding to the nearest possible number. These rounding rules normally don't allow us to see that "tiny precision loss", but it exists.
We can see this in action:
alert( 0.1.toFixed(20) ); // 0.10000000000000000555
And when we sum two numbers, their "precision losses" add up.
That's why 0.1 + 0.2 is not exactly 0.3.
The same issue exists in many other programming languages.
PHP, Java, C, Perl, Ruby give exactly the same result, because they are based on the same numeric format.
Can we work around the problem? Sure, the most reliable method is to round the result with the help of a method toFixed(n):
let sum = 0.1 + 0.2;
alert( sum.toFixed(2) ); // 0.30
Please note that toFixed always returns a string. It ensures that it has 2 digits after the decimal point. That's actually convenient if we have an e-shopping and need to show $0.30. For other cases, we can use the unary plus to coerce it into a number:
let sum = 0.1 + 0.2;
alert( +sum.toFixed(2) ); // 0.3
We also can temporarily multiply the numbers by 100 (or a bigger number) to turn them into integers, do the maths, and then divide back. Then, as we're doing maths with integers, the error somewhat decreases, but we still get it on division:
alert( (0.1 * 10 + 0.2 * 10) / 10 ); // 0.3
alert( (0.28 * 100 + 0.14 * 100) / 100); // 0.4200000000000001
So, multiply/divide approach reduces the error, but doesn't remove it totally.
Sometimes we could try to evade fractions at all. Like if we're dealing with a shop, then we can store prices in cents instead of dollars. But what if we apply a discount of 30%? In practice, totally evading fractions is rarely possible. Just round them to cut "tails" when needed.
Try running this:
```js run
// Hello! I'm a self-increasing number!
alert( 9999999999999999 ); // shows 10000000000000000
```
This suffers from the same issue: a loss of precision. There are 64 bits for the number, 52 of them can be used to store digits, but that's not enough. So the least significant digits disappear.
JavaScript doesn't trigger an error in such events. It does its best to fit the number into the desired format, but unfortunately, this format is not big enough.
Another funny consequence of the internal representation of numbers is the existence of two zeroes: `0` and `-0`.
That's because a sign is represented by a single bit, so it can be set or not set for any number including a zero.
In most cases the distinction is unnoticeable, because operators are suited to treat them as the same.
Remember these two special numeric values?
Infinity (and -Infinity) is a special numeric value that is greater (less) than anything.NaN represents an error.They belong to the type number, but are not "normal" numbers, so there are special functions to check for them:
isNaN(value) converts its argument to a number and then tests it for being NaN:
alert( isNaN(NaN) ); // true
alert( isNaN("str") ); // true
But do we need this function? Can't we just use the comparison === NaN? Sorry, but the answer is no. The value NaN is unique in that it does not equal anything, including itself:
alert( NaN === NaN ); // false
isFinite(value) converts its argument to a number and returns true if it's a regular number, not NaN/Infinity/-Infinity:
alert( isFinite("15") ); // true
alert( isFinite("str") ); // false, because a special value: NaN
alert( isFinite(Infinity) ); // false, because a special value: Infinity
Sometimes isFinite is used to validate whether a string value is a regular number:
let num = +prompt("Enter a number", '');
// will be true unless you enter Infinity, -Infinity or not a number
alert( isFinite(num) );
Please note that an empty or a space-only string is treated as 0 in all numeric functions including isFinite.
```smart header="Compare with Object.is"
There is a special built-in method Object.is that compares values like ===, but is more reliable for two edge cases:
NaN: Object.is(NaN, NaN) === true, that's a good thing.0 and -0 are different: Object.is(0, -0) === false, technically that's true, because internally the number has a sign bit that may be different even if all other bits are zeroes.In all other cases, Object.is(a, b) is the same as a === b.
This way of comparison is often used in JavaScript specification. When an internal algorithm needs to compare two values for being exactly the same, it uses Object.is (internally called SameValue).
## parseInt and parseFloat
Numeric conversion using a plus `+` or `Number()` is strict. If a value is not exactly a number, it fails:
```js run
alert( +"100px" ); // NaN
The sole exception is spaces at the beginning or at the end of the string, as they are ignored.
But in real life we often have values in units, like "100px" or "12pt" in CSS. Also in many countries the currency symbol goes after the amount, so we have "19€" and would like to extract a numeric value out of that.
That's what parseInt and parseFloat are for.
They "read" a number from a string until they can't. In case of an error, the gathered number is returned. The function parseInt returns an integer, whilst parseFloat will return a floating-point number:
alert( parseInt('100px') ); // 100
alert( parseFloat('12.5em') ); // 12.5
alert( parseInt('12.3') ); // 12, only the integer part is returned
alert( parseFloat('12.3.4') ); // 12.3, the second point stops the reading
There are situations when parseInt/parseFloat will return NaN. It happens when no digits could be read:
alert( parseInt('a123') ); // NaN, the first symbol stops the process
````smart header="The second argument of parseInt(str, radix)"
The parseInt() function has an optional second parameter. It specifies the base of the numeral system, so parseInt can also parse strings of hex numbers, binary numbers and so on:
alert( parseInt('0xff', 16) ); // 255
alert( parseInt('ff', 16) ); // 255, without 0x also works
alert( parseInt('2n9c', 36) ); // 123456
## Other math functions
JavaScript has a built-in [Math](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math) object which contains a small library of mathematical functions and constants.
A few examples:
`Math.random()`
: Returns a random number from 0 to 1 (not including 1).
```js run
alert( Math.random() ); // 0.1234567894322
alert( Math.random() ); // 0.5435252343232
alert( Math.random() ); // ... (any random numbers)
```
`Math.max(a, b, c...)` / `Math.min(a, b, c...)`
: Returns the greatest/smallest from the arbitrary number of arguments.
```js run
alert( Math.max(3, 5, -10, 0, 1) ); // 5
alert( Math.min(1, 2) ); // 1
```
`Math.pow(n, power)`
: Returns `n` raised to the given power.
```js run
alert( Math.pow(2, 10) ); // 2 in power 10 = 1024
```
There are more functions and constants in `Math` object, including trigonometry, which you can find in the [docs for the Math object](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math).
## Summary
To write numbers with many zeroes:
- Append `"e"` with the zeroes count to the number. Like: `123e6` is the same as `123` with 6 zeroes `123000000`.
- A negative number after `"e"` causes the number to be divided by 1 with given zeroes. E.g. `123e-6` means `0.000123` (`123` millionths).
For different numeral systems:
- Can write numbers directly in hex (`0x`), octal (`0o`) and binary (`0b`) systems.
- `parseInt(str, base)` parses the string `str` into an integer in numeral system with given `base`, `2 ≤ base ≤ 36`.
- `num.toString(base)` converts a number to a string in the numeral system with the given `base`.
For converting values like `12pt` and `100px` to a number:
- Use `parseInt/parseFloat` for the "soft" conversion, which reads a number from a string and then returns the value they could read before the error.
For fractions:
- Round using `Math.floor`, `Math.ceil`, `Math.trunc`, `Math.round` or `num.toFixed(precision)`.
- Make sure to remember there's a loss of precision when working with fractions.
More mathematical functions:
- See the [Math](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math) object when you need them. The library is very small, but can cover basic needs.