Number.prototype.tofixed()

Содержание:

Triggers

  $('.header').trigger('detach.ScrollToFixed'); // Removes scrollToFixed from the element.  The
                                                // namespace ensures remove will not be called
                                                // on other plugins that may be listening for
                                                // that event!  NOTE: Renamed as "detach" to
                                                // avoid the new Chrome native "remove" method.

  $('.header').trigger('resize'); // Resizes the spacer in case the fixed element height changes.
                                  // Good for size changes to the fixed element.

  $(window).scroll(); // Causes the plugin to recalculate the window scoll.
                      // Good for layout changes that could change the fixed element's response to
                      // the scroll.  Example: the fixed element height expands which should cause
                      // it to invoke its limit.

  $(window).resize(); // Causes the plugin to recalculate the element offsets, then the window scroll.
                      // Good for layout changes that could cause the fixed element to move.
                      // Example: the header height increases which should cause the fixed
                      // element to fix at a greater vertical scroll position.

Images

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Tests: isFinite and isNaN

Remember these two special numeric values?

(and ) is a special numeric value that is greater (less) than anything.
represents an error.

They belong to the type , but are not “normal” numbers, so there are special functions to check for them:

converts its argument to a number and then tests it for being :

But do we need this function? Can’t we just use the comparison ? Sorry, but the answer is no. The value is unique in that it does not equal anything, including itself:
converts its argument to a number and returns if it’s a regular number, not :

Sometimes is used to validate whether a string value is a regular number:

Please note that an empty or a space-only string is treated as in all numeric functions including .

Compare with

There is a special built-in method that compares values like , but is more reliable for two edge cases:

It works with : , that’s a good thing.
Values and are different: , 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, is the same as .

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 (internally called ).

JavaScript

JS Array
concat()
constructor
copyWithin()
entries()
every()
fill()
filter()
find()
findIndex()
forEach()
from()
includes()
indexOf()
isArray()
join()
keys()
length
lastIndexOf()
map()
pop()
prototype
push()
reduce()
reduceRight()
reverse()
shift()
slice()
some()
sort()
splice()
toString()
unshift()
valueOf()

JS Boolean
constructor
prototype
toString()
valueOf()

JS Classes
constructor()
extends
static
super

JS Date
constructor
getDate()
getDay()
getFullYear()
getHours()
getMilliseconds()
getMinutes()
getMonth()
getSeconds()
getTime()
getTimezoneOffset()
getUTCDate()
getUTCDay()
getUTCFullYear()
getUTCHours()
getUTCMilliseconds()
getUTCMinutes()
getUTCMonth()
getUTCSeconds()
now()
parse()
prototype
setDate()
setFullYear()
setHours()
setMilliseconds()
setMinutes()
setMonth()
setSeconds()
setTime()
setUTCDate()
setUTCFullYear()
setUTCHours()
setUTCMilliseconds()
setUTCMinutes()
setUTCMonth()
setUTCSeconds()
toDateString()
toISOString()
toJSON()
toLocaleDateString()
toLocaleTimeString()
toLocaleString()
toString()
toTimeString()
toUTCString()
UTC()
valueOf()

JS Error
name
message

JS Global
decodeURI()
decodeURIComponent()
encodeURI()
encodeURIComponent()
escape()
eval()
Infinity
isFinite()
isNaN()
NaN
Number()
parseFloat()
parseInt()
String()
undefined
unescape()

JS JSON
parse()
stringify()

JS Math
abs()
acos()
acosh()
asin()
asinh()
atan()
atan2()
atanh()
cbrt()
ceil()
cos()
cosh()
E
exp()
floor()
LN2
LN10
log()
LOG2E
LOG10E
max()
min()
PI
pow()
random()
round()
sin()
sqrt()
SQRT1_2
SQRT2
tan()
tanh()
trunc()

JS Number
constructor
isFinite()
isInteger()
isNaN()
isSafeInteger()
MAX_VALUE
MIN_VALUE
NEGATIVE_INFINITY
NaN
POSITIVE_INFINITY
prototype
toExponential()
toFixed()
toLocaleString()
toPrecision()
toString()
valueOf()

JS OperatorsJS RegExp
constructor
compile()
exec()
g
global
i
ignoreCase
lastIndex
m
multiline
n+
n*
n?
n{X}
n{X,Y}
n{X,}
n$
^n
?=n
?!n
source
test()
toString()

(x|y)
.
\w
\W
\d
\D
\s
\S
\b
\B
\0
\n
\f
\r
\t
\v
\xxx
\xdd
\uxxxx

JS Statements
break
class
continue
debugger
do…while
for
for…in
for…of
function
if…else
return
switch
throw
try…catch
var
while

JS String
charAt()
charCodeAt()
concat()
constructor
endsWith()
fromCharCode()
includes()
indexOf()
lastIndexOf()
length
localeCompare()
match()
prototype
repeat()
replace()
search()
slice()
split()
startsWith()
substr()
substring()
toLocaleLowerCase()
toLocaleUpperCase()
toLowerCase()
toString()
toUpperCase()
trim()
valueOf()

Exemples

var numObj = 12345.6789;

numObj.toFixed();       // Renvoie '12346' : arrondi, aucune partie fractionnaire
numObj.toFixed(1);      // Renvoie '12345.7' : arrondi ici aussi
numObj.toFixed(6);      // Renvoie '12345.678900' : des zéros sont ajoutés
(1.23e+20).toFixed(2);  // Renvoie '123000000000000000000.00'
(1.23e-10).toFixed(2);  // Renvoie '0.00'
2.34.toFixed(1);        // Renvoie '2.3'
-2.34.toFixed(1);       // Renvoie -2.3 (en raison de la précédence des opérateurs, 
                        // les littéraux de nombres négatifs ne renvoient pas de chaînes)
2.35.toFixed(1);        // Renvoie '2.4' (arrondi supérieur)
2.55.toFixed(1);        // Renvoie '2.5' (cf. l'avertissement ci-avant)
(-2.34).toFixed(1);     // Renvoie '-2.3'

Compatibilité des navigateurs

Le tableau de compatibilité de cette page a été généré à partir de données structurées. Si vous souhaitez contribuer à ces données, n’hésitez pas à envoyer une pull request sur https://github.com/mdn/browser-compat-data.

Update compatibility data on GitHub

	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Webview Android	Chrome pour Android	Firefox pour Android	Opera pour Android	Safari sur iOS	Samsung Internet	Node.js
Chrome Support complet 1	Edge Support complet 12	Firefox Support complet 1	IE Support complet 5.5	Opera Support complet 7	Safari Support complet 2	WebView Android Support complet 1	Chrome Android Support complet 18	Firefox Android Support complet 4	Opera Android Support complet 10.1	Safari iOS Support complet 1	Samsung Internet Android Support complet 1.0	nodejs Support complet 0.1.100

More Examples

Example

A cross-browser solution (using scrollLeft and scrollTop for IE8 and
earlier):

window.scrollBy(100, 100);if (window.pageXOffset !== undefined) { // All browsers, except IE9 and earlier alert(window.pageXOffset + window.pageYOffset);} else { // IE9 and earlier alert(document.documentElement.scrollLeft + document.documentElement.scrollTop);}

Example

Create a sticky navigation bar:

// Get the navbarvar navbar = document.getElementById(«navbar»);//
Get the offset position of the navbarvar sticky = navbar.offsetTop;// Add the sticky class to the navbar when you reach its scroll position. Remove the sticky class when you leave the scroll position.function myFunction() { if (window.pageYOffset
>= sticky) { navbar.classList.add(«sticky») }
else { navbar.classList.remove(«sticky»); }
}

❮ Window Object

Description

is a top-level function and not a method of any object.

If encounters a character other than a plus sign (), minus sign ( U+002D HYPHEN-MINUS), numeral (–), decimal point (), or exponent ( or ), it returns the value up to that character, ignoring the invalid character and characters following it.
A second decimal point also stops parsing (characters up to that point will still be parsed).
Leading and trailing spaces in the argument are ignored.
If the argument’s first character can’t be converted to a number (it’s not any of the above characters), returns .
can also parse and return .
converts syntax to , losing precision. This happens because the trailing character is discarded.

Consider for stricter parsing, which converts to for arguments with invalid characters anywhere.

will parse non-string objects if they have a or method. The returned value is the same as if had been called on the result of those methods.

Паттерн Prototype

Object.create(), метод Constructor, и class создают объекты на основе системы прототипа.

Рассмотрим следующий пример:

let service = { doSomething : function() {}}let specializedService = Object.create(service);console.log(specializedService.__proto__ === service); //true

Я использовал Object.create () для создания нового объекта specializedService, прототипом которого является служебный объект. Это означает, что doSomething () доступен на specializedService объекте. Это также означает, что свойство __proto__ объекта specializedService указывает на служебный объект.

Теперь построим аналогичный объект с помощью класса:

class Service { doSomething(){}}class SpecializedService extends Service {}let specializedService = new SpecializedService();console.log(specializedService.__proto__ === SpecializedService.prototype);

Все методы, определенные в классе Service будут добавлены к объекту Service.prototype. Все экземпляры класса Service будут иметь один и тот же прототип (Service.prototype) объекта. Все экземпляры класса делегируют вызов методов объекту Service.prototype. Методы всего лишь один раз определяются на Service.prototype, а затем наследуются всеми экземплярами класса.

JavaScript

JS Boolean
constructor
prototype
toString()
valueOf()

JS Classes
constructor()
extends
static
super

JS Error
name
message

JS Global
decodeURI()
decodeURIComponent()
encodeURI()
encodeURIComponent()
escape()
eval()
Infinity
isFinite()
isNaN()
NaN
Number()
parseFloat()
parseInt()
String()
undefined
unescape()

JS JSON
parse()
stringify()

JS OperatorsJS RegExp
constructor
compile()
exec()
g
global
i
ignoreCase
lastIndex
m
multiline
n+
n*
n?
n{X}
n{X,Y}
n{X,}
n$
^n
?=n
?!n
source
test()
toString()

(x|y)
.
\w
\W
\d
\D
\s
\S
\b
\B
\0
\n
\f
\r
\t
\v
\xxx
\xdd
\uxxxx

JS Statements
break
class
continue
debugger
do…while
for
for…in
for…of
function
if…else
return
switch
throw
try…catch
var
while

Imprecise calculations

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:

What may be a little less obvious, but happens quite often, is the loss of precision.

Consider this (falsy!) test:

That’s right, if we check whether the sum of and is , we get .

Strange! What is it then if not ?

Ouch! There are more consequences than an incorrect comparison here. Imagine you’re making an e-shopping site and the visitor puts and goods into their cart. The order total will be . 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 , that look simple in the decimal numeric system are actually unending fractions in their binary form.

In other words, what is ? It is one divided by ten , one-tenth. In decimal numeral system such numbers are easily representable. Compare it to one-third: . It becomes an endless fraction .

So, division by powers is guaranteed to work well in the decimal system, but division by is not. For the same reason, in the binary numeral system, the division by powers of is guaranteed to work, but 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:

And when we sum two numbers, their “precision losses” add up.

That’s why is not exactly .

Not only JavaScript

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):

Please note that 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 . For other cases, we can use the unary plus to coerce it into a number:

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:

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.

The funny thing

Try running this:

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.

Two zeroes

Another funny consequence of the internal representation of numbers is the existence of two zeroes: and .

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.

Menus

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Объекты

Объект представляет собой динамический набор пар ключ-значение. Ключ — это строка. Значение может быть примитивом, объектом или функцией.

Самый простой способ создать объект — использовать литерал объекта:

let obj = {  message : "A message",  doSomething : function() {}}

Мы можем читать, добавлять, редактировать и удалять свойства объекта в любое время.

get: ,
set:
delete: ,

let obj = {}; //create empty objectobj.message = "A message"; //add propertyobj.message = "A new message"; //edit propertydelete object.message; //delete property

Объекты реализуются как хэш-карты. Простую хэш-карту можно создать с помощью Object.create (null):

let french = Object.create(null);french = "oui";french  = "non";french;//"oui"

Если вы хотите сделать объект неизменным, используйте Object.freeze () .

Object.keys () может использоваться для перебора всех свойств.

function logProperty(name){  console.log(name); //property name  console.log(obj); //property value}Object.keys(obj).forEach(logProperty);

More ways to write a number

Imagine we need to write 1 billion. The obvious way is:

But in real life, we usually avoid writing a long string of zeroes as it’s easy to mistype. Also, we are lazy. We will usually write something like for a billion or for 7 billion 300 million. The same is true for most large numbers.

In JavaScript, we shorten a number by appending the letter to the number and specifying the zeroes count:

In other words, multiplies the number by with the given zeroes count.

Now let’s write something very small. Say, 1 microsecond (one millionth of a second):

Just like before, using can help. If we’d like to avoid writing the zeroes explicitly, we could say the same as:

If we count the zeroes in , there are 6 of them. So naturally it’s .

In other words, a negative number after means a division by 1 with the given number of zeroes:

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: and then the number.

For instance:

Binary and octal numeral systems are rarely used, but also supported using the and prefixes:

There are only 3 numeral systems with such support. For other numeral systems, we should use the function (which we will see later in this chapter).