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Эта глава описывает предопределённые объекты в стандартном JavaScript: Array
, Boolean
, Date
, Function
, Math
, Number
, RegExp
, и String
.
Объект Array
В JavaScript массив как тип данных отсутствует. Тем не менее, вы можете использовать предопределённый объект Array
и его методы, чтобы работать с массивами в ваших приложениях. Объект Array
содержит различные методы для манипуляций с массивами, такими как объединение, обращение и сортировка. У него есть свойство для определения длины массива и другие свойства для работы с регулярными выражениями.
Массив - это упорядоченный набор значений, обращаться к которым можно с помощью имени и индекса. Например, можно создать массив emp
, который содержит имена сотрудников фирмы, проиндексированные номерами. Тогда emp[1]
будет соответствовать сотруднику номер один, emp[2]
сотруднику номер два и так далее.
Создание объекта Array
Следующие инструкции создают эквивалентные массивы:
var arr = new Array(element0, element1, ..., elementN); var arr = Array(element0, element1, ..., elementN); var arr = [element0, element1, ..., elementN];
element0, element1, ..., elementN
это список значений во вновь создаваемом массиве. Когда эти значения заданы, массив инициализирует ими свои эелементы. Длина массива определяется по числу аргументов и сохраняется в свойстве length
(длина).
Синтаксис с квадратными скобками называется "литералом массива" (array literal) или "инициализатором массива" (array initializer). Такая запись короче других и используется чаще. Подробности смотрите в Array Literals.
Создать массив ненулевой длины, но без элементов, можно одним из следующих способов:
var arr = new Array(arrayLength); var arr = Array(arrayLength); // This has exactly the same effect var arr = []; arr.length = arrayLength;
Обратите внимание: в приведённом выше коде arrayLength
должен быть значением типа Number
, то есть числом, иначе будет создан массив с одним элементом (переданным конструктору значением). Вызов arr.length
возвратит arrayLength
, хотя на самом деле элемент содержит пустые (неопределённые, undefined) элементы. Итерация по массиву циклом for...in не возвратит никаких элементов.
Массивы можно не только определить как и переменные, но и присвоить существующему объекту как свойство:
var obj = {}; // ... obj.prop = [element0, element1, ..., elementN]; // OR var obj = {prop: [element0, element1, ...., elementN]}
Если же вы хотите создать одноэлементный массив, содержащий число, придётся использовать запись с квадратными скобками, так как когда конструктору Array() передаётся одно-единственное число, оно трактуется как длина массива, а не как хранимвый элемент.
var arr = [42];
var arr = Array(42); // Creates an array with no element, but with arr.length set to 42
// The above code is equivalent to
var arr = [];
arr.length = 42;
Вызов Array(N)
приводит к RangeError
, если N
не является целым числом, чья дробная часть не равна нулю. Следующий пример иллюстрирует это поведение.
var arr = Array(9.3); // RangeError: Invalid array length
Если ваш код должен создавать массивы с отдельными элементами произвольного типа, безопаснее использовать литералы массива. Или, создайте пустой массив, прежде чем добавлять элементы к нему.
Заполнение массива
You can populate an array by assigning values to its elements. For example,
var emp = []; emp[0] = "Casey Jones"; emp[1] = "Phil Lesh"; emp[2] = "August West";
Note: if you supply a non-integer value to the array operator in the code above, a property will be created in the object representing the array, instead of an array element.
var arr = []; arr[3.4] = "Oranges"; console.log(arr.length); // 0 console.log(arr.hasOwnProperty(3.4)); // true
You can also populate an array when you create it:
var myArray = new Array("Hello", myVar, 3.14159); var myArray = ["Mango", "Apple", "Orange"]
Referring to Array Elements
You refer to an array's elements by using the element's ordinal number. For example, suppose you define the following array:
var myArray = ["Wind", "Rain", "Fire"];
You then refer to the first element of the array as myArray[0]
and the second element of the array as myArray[1]
. The index of the elements begins with zero.
Note: the array operator (square brackets) is also used for accessing the array's properties (arrays are also objects in JavaScript). For example,
var arr = ["one", "two", "three"]; arr[2]; // three arr["length"]; // 3
Understanding length
At the implementation level, JavaScript's arrays actually store their elements as standard object properties, using the array index as the property name. The length
property is special; it always returns the index of the last element plus one (in following example Dusty is indexed at 30 so cats.length returns 30 + 1). Remember, Javascript Array indexes are 0-based: they start at 0, not 1. This means that the
property will be one more than the highest index stored in the array:length
var cats = []; cats[30] = ['Dusty']; print(cats.length); // 31
You can also assign to the length
property. Writing a value that is shorter than the number of stored items truncates the array; writing 0 empties it entirely:
var cats = ['Dusty', 'Misty', 'Twiggy']; console.log(cats.length); // 3 cats.length = 2; console.log(cats); // prints "Dusty,Misty" - Twiggy has been removed cats.length = 0; console.log(cats); // prints nothing; the cats array is empty cats.length = 3; console.log(cats); // [undefined, undefined, undefined]
Iterating over arrays
A common operation is to iterate over the values of an array, processing each one in some way. The simplest way to do this is as follows:
var colors = ['red', 'green', 'blue']; for (var i = 0; i < colors.length; i++) { console.log(colors[i]); }
If you know that none of the elements in your array evaluate to false
in a boolean context — if your array consists only of DOM nodes, for example, you can use a more efficient idiom:
var divs = document.getElementsByTagName('div'); for (var i = 0, div; div = divs[i]; i++) { /* Process div in some way */ }
This avoids the overhead of checking the length of the array, and ensures that the div
variable is reassigned to the current item each time around the loop for added convenience.
Introduced in JavaScript 1.6
The forEach()
method, introduced in JavaScript 1.6, provides another way of iterating over an array:
var colors = ['red', 'green', 'blue']; colors.forEach(function(color) { console.log(color); });
The function passed to forEach
is executed once for every item in the array, with the array item passed as the argument to the function. Unassigned values are not iterated in a forEach
loop.
Note that the elements of array omitted when the array is defined are not listed when iterating by forEach,
but are listed when undefined
has been manually assigned to the element:
var array = ['first', 'second', , 'fourth']; // returns ['first', 'second', 'fourth']; array.forEach(function(element) { console.log(element); }) if(array[2] === undefined) { console.log('array[2] is undefined'); } // true var array = ['first', 'second', undefined, 'fourth']; // returns ['first', 'second', undefined, 'fourth']; array.forEach(function(element) { console.log(element); })
Since JavaScript elements are saved as standard object properties, it is not advisable to iterate through JavaScript arrays using for...in loops because normal elements and all enumerable properties will be listed.
Array Methods
The Array
object has the following methods:
concat()
joins two arrays and returns a new array.var myArray = new Array("1", "2", "3"); myArray = myArray.concat("a", "b", "c"); // myArray is now ["1", "2", "3", "a", "b", "c"]
join(deliminator = ",")
joins all elements of an array into a string.var myArray = new Array("Wind", "Rain", "Fire"); var list = myArray.join(" - "); // list is "Wind - Rain - Fire"
push()
adds one or more elements to the end of an array and returns the resulting length of the array.var myArray = new Array("1", "2"); myArray.push("3"); // myArray is now ["1", "2", "3"]
pop()
removes the last element from an array and returns that element.var myArray = new Array("1", "2", "3"); var last = myArray.pop(); // myArray is now ["1", "2"], last = "3"
shift()
removes the first element from an array and returns that element.var myArray = new Array ("1", "2", "3"); var first = myArray.shift(); // myArray is now ["2", "3"], first is "1"
unshift()
adds one or more elements to the front of an array and returns the new length of the array.var myArray = new Array ("1", "2", "3"); myArray.unshift("4", "5"); // myArray becomes ["4", "5", "1", "2", "3"]
slice(start_index, upto_index)
extracts a section of an array and returns a new array.var myArray = new Array ("a", "b", "c", "d", "e"); myArray = myArray.slice(1, 4); /* starts at index 1 and extracts all elements until index 3, returning [ "b", "c", "d"] */
splice(index, count_to_remove, addelement1, addelement2, ...)
removes elements from an array and (optionally) replaces them.var myArray = new Array ("1", "2", "3", "4", "5"); myArray.splice(1, 3, "a", "b", "c", "d"); // myArray is now ["1", "a", "b", "c", "d", "5"] // This code started at index one (or where the "2" was), removed 3 elements there, // and then inserted all consecutive elements in its place.
reverse()
transposes the elements of an array: the first array element becomes the last and the last becomes the first.var myArray = new Array ("1", "2", "3"); myArray.reverse(); // transposes the array so that myArray = [ "3", "2", "1" ]
sort()
sorts the elements of an array.var myArray = new Array("Wind", "Rain", "Fire"); myArray.sort(); // sorts the array so that myArrray = [ "Fire", "Rain", "Wind" ]
sort()
can also take a callback function to determine how array elements are compared. The function compares two values and returns one of three values:- if
a
is less thanb
by the sorting system, return -1 (or any negative number) - if
a
is greater thanb
by the sorting system, return 1 (or any positive number) - if
a
andb
are considered equivalent, return 0.
For instance, the following will sort by the last letter of an array:
var sortFn = function(a, b){ if (a[a.length - 1] < b[b.length - 1]) return -1; if (a[a.length - 1] > b[b.length - 1]) return 1; if (a[a.length - 1] == b[b.length - 1]) return 0; } myArray.sort(sortFn); // sorts the array so that myArray = ["Wind","Fire","Rain"]
- if
Introduced in JavaScript 1.6
Compatibility code for older browsers can be found for each of these functions on the individual pages. Native browser support for these features in various browsers can be found here.
indexOf(searchElement[, fromIndex])
searches the array forsearchElement
and returns the index of the first match.var a = ['a', 'b', 'a', 'b', 'a']; alert(a.indexOf('b')); // Alerts 1 // Now try again, starting from after the last match alert(a.indexOf('b', 2)); // Alerts 3 alert(a.indexOf('z')); // Alerts -1, because 'z' was not found
lastIndexOf(searchElement[, fromIndex])
works likeindexOf
, but starts at the end and searches backwards.var a = ['a', 'b', 'c', 'd', 'a', 'b']; alert(a.lastIndexOf('b')); // Alerts 5 // Now try again, starting from before the last match alert(a.lastIndexOf('b', 4)); // Alerts 1 alert(a.lastIndexOf('z')); // Alerts -1
forEach(callback[, thisObject])
executescallback
on every array item.var a = ['a', 'b', 'c']; a.forEach(alert); // Alerts each item in turn
map(callback[, thisObject])
returns a new array of the return value from executingcallback
on every array item.var a1 = ['a', 'b', 'c']; var a2 = a1.map(function(item) { return item.toUpperCase(); }); alert(a2); // Alerts A,B,C
filter(callback[, thisObject])
returns a new array containing the items for which callback returned true.var a1 = ['a', 10, 'b', 20, 'c', 30]; var a2 = a1.filter(function(item) { return typeof item == 'number'; }); alert(a2); // Alerts 10,20,30
every(callback[, thisObject])
returns true ifcallback
returns true for every item in the array.function isNumber(value){ return typeof value == 'number'; } var a1 = [1, 2, 3]; alert(a1.every(isNumber)); // Alerts true var a2 = [1, '2', 3]; alert(a2.every(isNumber)); // Alerts false
some(callback[, thisObject])
returns true ifcallback
returns true for at least one item in the array.function isNumber(value){ return typeof value == 'number'; } var a1 = [1, 2, 3]; alert(a1.some(isNumber)); // Alerts true var a2 = [1, '2', 3]; alert(a2.some(isNumber)); // Alerts true var a3 = ['1', '2', '3']; alert(a3.some(isNumber)); // Alerts false
The methods above that take a callback are known as iterative methods, because they iterate over the entire array in some fashion. Each one takes an optional second argument called thisObject
. If provided, thisObject
becomes the value of the this
keyword inside the body of the callback function. If not provided, as with other cases where a function is invoked outside of an explicit object context, this
will refer to the global object (window
).
The callback function is actually called with three arguments. The first is the value of the current item, the second is its array index, and the third is a reference to the array itself. JavaScript functions ignore any arguments that are not named in the parameter list so it is safe to provide a callback function that only takes a single argument, such as alert
.
Introduced in JavaScript 1.8
reduce(callback[, initialValue])
appliescallback(firstValue, secondValue)
to reduce the list of items down to a single value.var a = [10, 20, 30]; var total = a.reduce(function(first, second) { return first + second; }, 0); alert(total) // Alerts 60
reduceRight(callback[, initialValue])
works likereduce()
, but starts with the last element.
reduce
and reduceRight
are the least obvious of the iterative array methods. They should be used for algorithms that combine two values recursively in order to reduce a sequence down to a single value.
Multi-Dimensional Arrays
Arrays can be nested, meaning that an array can contain another array as an element. Using this characteristic of JavaScript arrays, multi-dimensional arrays can be created.
The following code creates a two-dimensional array.
var a = new Array(4); for (i = 0; i < 4; i++) { a[i] = new Array(4); for (j = 0; j < 4; j++) { a[i][j] = "[" + i + "," + j + "]"; } }
This example creates an array with the following rows:
Row 0: [0,0] [0,1] [0,2] [0,3] Row 1: [1,0] [1,1] [1,2] [1,3] Row 2: [2,0] [2,1] [2,2] [2,3] Row 3: [3,0] [3,1] [3,2] [3,3]
Arrays and Regular Expressions
When an array is the result of a match between a regular expression and a string, the array returns properties and elements that provide information about the match. An array is the return value of RegExp.exec()
, String.match()
, and String.split()
. For information on using arrays with regular expressions, see Regular Expressions.
Working with Array-like objects
Introduced in JavaScript 1.6
Some JavaScript objects, such as the NodeList
returned by document.getElementsByTagName()
or the arguments
object made available within the body of a function, look and behave like arrays on the surface but do not share all of their methods. The arguments
object provides a length
attribute but does not implement the forEach()
method, for example.
Array generics, introduced in JavaScript 1.6, provide a way of running Array
methods against other array-like objects. Each standard array method has a corresponding method on the Array
object itself; for example:
function alertArguments() { Array.forEach(arguments, function(item) { alert(item); }); }
These generic methods can be emulated more verbosely in older versions of JavaScript using the call method provided by JavaScript function objects:
Array.prototype.forEach.call(arguments, function(item) { alert(item); });
Array generic methods can be used on strings as well, since they provide sequential access to their characters in a similar way to arrays:
Array.forEach("a string", function(chr) { alert(chr); });
Here are some further examples of applying array methods to strings, also taking advantage of JavaScript 1.8 expression closures:
var str = 'abcdef'; var consonantsOnlyStr = Array.filter(str, function (c) !(/[aeiou]/i).test(c)).join(''); // 'bcdf' var vowelsPresent = Array.some(str, function (c) (/[aeiou]/i).test(c)); // true var allVowels = Array.every(str, function (c) (/[aeiou]/i).test(c)); // false var interpolatedZeros = Array.map(str, function (c) c+'0').join(''); // 'a0b0c0d0e0f0' var numerologicalValue = Array.reduce(str, function (c, c2) c+c2.toLowerCase().charCodeAt()-96, 0); // 21 (reduce() since JS v1.8)
Note that filter
and map
do not automatically return the characters back into being members of a string in the return result; an array is returned, so we must use join
to return back to a string.
Array comprehensions
Introduced in JavaScript 1.7
Introduced in JavaScript 1.7, array comprehensions provide a useful shortcut for constructing a new array based on the contents of another. Comprehensions can often be used in place of calls to map()
and filter()
, or as a way of combining the two.
The following comprehension takes an array of numbers and creates a new array of the double of each of those numbers.
var numbers = [1, 2, 3, 4]; var doubled = [i * 2 for (i of numbers)]; alert(doubled); // Alerts 2,4,6,8
This is equivalent to the following map()
operation:
var doubled = numbers.map(function(i){return i * 2;});
Comprehensions can also be used to select items that match a particular expression. Here is a comprehension which selects only even numbers:
var numbers = [1, 2, 3, 21, 22, 30]; var evens = [i for (i of numbers) if (i % 2 === 0)]; alert(evens); // Alerts 2,22,30
filter()
can be used for the same purpose:
var evens = numbers.filter(function(i){return i % 2 === 0;});
map()
and filter()
style operations can be combined into a single array comprehension. Here is one that filters just the even numbers, then creates an array containing their doubles:
var numbers = [1, 2, 3, 21, 22, 30]; var doubledEvens = [i * 2 for (i of numbers) if (i % 2 === 0)]; alert(doubledEvens); // Alerts 4,44,60
The square brackets of an array comprehension introduce an implicit block for scoping purposes. New variables (such as i in the example) are treated as if they had been declared using let
. This means that they will not be available outside of the comprehension.
The input to an array comprehension does not itself need to be an array; iterators and generators can also be used.
Even strings may be used as input; to achieve the filter and map actions (under Array-like objects) above:
var str = 'abcdef'; var consonantsOnlyStr = [c for (c of str) if (!(/[aeiouAEIOU]/).test(c)) ].join(''); // 'bcdf' var interpolatedZeros = [c+'0' for (c of str) ].join(''); // 'a0b0c0d0e0f0'
Again, the input form is not preserved, so we have to use join()
to revert back to a string.
Boolean Object
The Boolean
object is a wrapper around the primitive Boolean data type. Use the following syntax to create a Boolean
object:
var booleanObjectName = new Boolean(value);
Do not confuse the primitive Boolean values true
and false
with the true and false values of the Boolean
object. Any object whose value is not undefined
, null
, 0
, NaN
, or the empty string, including a Boolean
object whose value is false, evaluates to true when passed to a conditional statement. See if...else Statement for more information.
Date Object
JavaScript does not have a date data type. However, you can use the Date
object and its methods to work with dates and times in your applications. The Date
object has a large number of methods for setting, getting, and manipulating dates. It does not have any properties.
JavaScript handles dates similarly to Java. The two languages have many of the same date methods, and both languages store dates as the number of milliseconds since January 1, 1970, 00:00:00.
The Date
object range is -100,000,000 days to 100,000,000 days relative to 01 January, 1970 UTC.
To create a Date
object:
var dateObjectName = new Date([parameters]);
where dateObjectName
is the name of the Date
object being created; it can be a new object or a property of an existing object.
Calling Date
without the new
keyword simply converts the provided date to a string representation.
The parameters
in the preceding syntax can be any of the following:
- Nothing: creates today's date and time. For example,
today = new Date();
. - A string representing a date in the following form: "Month day, year hours:minutes:seconds." For example,
var Xmas95 = new Date("December 25, 1995 13:30:00")
. If you omit hours, minutes, or seconds, the value will be set to zero. - A set of integer values for year, month, and day. For example,
var Xmas95 = new Date(1995, 11, 25)
. - A set of integer values for year, month, day, hour, minute, and seconds. For example,
var Xmas95 = new Date(1995, 11, 25, 9, 30, 0);
.
JavaScript 1.2 and earlier
The Date
object behaves as follows:
- Dates prior to 1970 are not allowed.
- JavaScript depends on platform-specific date facilities and behavior; the behavior of the
Date
object varies from platform to platform.
Methods of the Date Object
The Date
object methods for handling dates and times fall into these broad categories:
- "set" methods, for setting date and time values in
Date
objects. - "get" methods, for getting date and time values from
Date
objects. - "to" methods, for returning string values from
Date
objects. - parse and UTC methods, for parsing
Date
strings.
With the "get" and "set" methods you can get and set seconds, minutes, hours, day of the month, day of the week, months, and years separately. There is a getDay
method that returns the day of the week, but no corresponding setDay
method, because the day of the week is set automatically. These methods use integers to represent these values as follows:
- Seconds and minutes: 0 to 59
- Hours: 0 to 23
- Day: 0 (Sunday) to 6 (Saturday)
- Date: 1 to 31 (day of the month)
- Months: 0 (January) to 11 (December)
- Year: years since 1900
For example, suppose you define the following date:
var Xmas95 = new Date("December 25, 1995");
Then Xmas95.getMonth()
returns 11, and Xmas95.getFullYear()
returns 1995.
The getTime
and setTime
methods are useful for comparing dates. The getTime
method returns the number of milliseconds since January 1, 1970, 00:00:00 for a Date
object.
For example, the following code displays the number of days left in the current year:
var today = new Date(); var endYear = new Date(1995, 11, 31, 23, 59, 59, 999); // Set day and month endYear.setFullYear(today.getFullYear()); // Set year to this year var msPerDay = 24 * 60 * 60 * 1000; // Number of milliseconds per day var daysLeft = (endYear.getTime() - today.getTime()) / msPerDay; var daysLeft = Math.round(daysLeft); //returns days left in the year
This example creates a Date
object named today
that contains today's date. It then creates a Date
object named endYear
and sets the year to the current year. Then, using the number of milliseconds per day, it computes the number of days between today
and endYear
, using getTime
and rounding to a whole number of days.
The parse
method is useful for assigning values from date strings to existing Date
objects. For example, the following code uses parse
and setTime
to assign a date value to the IPOdate
object:
var IPOdate = new Date(); IPOdate.setTime(Date.parse("Aug 9, 1995"));
Using the Date Object: an Example
In the following example, the function JSClock()
returns the time in the format of a digital clock.
function JSClock() { var time = new Date(); var hour = time.getHours(); var minute = time.getMinutes(); var second = time.getSeconds(); var temp = "" + ((hour > 12) ? hour - 12 : hour); if (hour == 0) temp = "12"; temp += ((minute < 10) ? ":0" : ":") + minute; temp += ((second < 10) ? ":0" : ":") + second; temp += (hour >= 12) ? " P.M." : " A.M."; return temp; }
The JSClock
function first creates a new Date
object called time
; since no arguments are given, time is created with the current date and time. Then calls to the getHours
, getMinutes
, and getSeconds
methods assign the value of the current hour, minute, and second to hour
, minute
, and second
.
The next four statements build a string value based on the time. The first statement creates a variable temp
, assigning it a value using a conditional expression; if hour
is greater than 12, (hour - 12
), otherwise simply hour, unless hour is 0, in which case it becomes 12.
The next statement appends a minute
value to temp
. If the value of minute
is less than 10, the conditional expression adds a string with a preceding zero; otherwise it adds a string with a demarcating colon. Then a statement appends a seconds value to temp
in the same way.
Finally, a conditional expression appends "P.M." to temp
if hour
is 12 or greater; otherwise, it appends "A.M." to temp
.
Function Object
The predefined Function
object specifies a string of JavaScript code to be compiled as a function.
To create a Function
object:
var functionObjectName = new Function ([arg1, arg2, ... argn], functionBody);
functionObjectName
is the name of a variable or a property of an existing object. It can also be an object followed by a lowercase event handler name, such as window.onerror
.
arg1
, arg2
, ... argn
are arguments to be used by the function as formal argument names. Each must be a string that corresponds to a valid JavaScript identifier; for example "x" or "theForm".
functionBody
is a string specifying the JavaScript code to be compiled as the function body.
Function
objects are evaluated each time they are used. This is less efficient than declaring a function and calling it within your code, because declared functions are compiled.
In addition to defining functions as described here, you can also use the function
statement and the function expression. See the JavaScript Reference for more information.
The following code assigns a function to the variable setBGColor
. This function sets the current document's background color.
var setBGColor = new Function("document.bgColor = 'antiquewhite'");
To call the Function
object, you can specify the variable name as if it were a function. The following code executes the function specified by the setBGColor
variable:
var colorChoice="antiquewhite"; if (colorChoice=="antiquewhite") {setBGColor()}
You can assign the function to an event handler in either of the following ways:
-
document.form1.colorButton.onclick = setBGColor;
-
<INPUT NAME="colorButton" TYPE="button" VALUE="Change background color" onClick="setBGColor()">
Creating the variable setBGColor
shown above is similar to declaring the following function:
function setBGColor() { document.bgColor = 'antiquewhite'; }
Assigning a function to a variable is similar to declaring a function, but there are differences:
- When you assign a function to a variable using
var setBGColor = new Function("...")
,setBGColor
is a variable for which the current value is a reference to the function created withnew Function()
. - When you create a function using
function setBGColor() {...}
,setBGColor
is not a variable, it is the name of a function.
You can nest a function within a function. The nested (inner) function is private to its containing (outer) function:
- The inner function can be accessed only from statements in the outer function.
- The inner function can use the arguments and variables of the outer function. The outer function cannot use the arguments and variables of the inner function.
Math Object
The predefined Math
object has properties and methods for mathematical constants and functions. For example, the Math
object's PI
property has the value of pi (3.141...), which you would use in an application as
Math.PI
Similarly, standard mathematical functions are methods of Math
. These include trigonometric, logarithmic, exponential, and other functions. For example, if you want to use the trigonometric function sine, you would write
Math.sin(1.56)
Note that all trigonometric methods of Math
take arguments in radians.
The following table summarizes the Math
object's methods.
Method | Description |
---|---|
abs |
Absolute value |
sin , cos , tan |
Standard trigonometric functions; argument in radians |
acos , asin , atan , atan2 |
Inverse trigonometric functions; return values in radians |
exp , log |
Exponential and natural logarithm, base e |
ceil |
Returns least integer greater than or equal to argument |
floor |
Returns greatest integer less than or equal to argument |
min , max |
Returns greater or lesser (respectively) of two arguments |
pow |
Exponential; first argument is base, second is exponent |
random |
Returns a random number between 0 and 1. |
round |
Rounds argument to nearest integer |
sqrt |
Square root |
Unlike many other objects, you never create a Math
object of your own. You always use the predefined Math
object.
Number Object
The Number
object has properties for numerical constants, such as maximum value, not-a-number, and infinity. You cannot change the values of these properties and you use them as follows:
var biggestNum = Number.MAX_VALUE; var smallestNum = Number.MIN_VALUE; var infiniteNum = Number.POSITIVE_INFINITY; var negInfiniteNum = Number.NEGATIVE_INFINITY; var notANum = Number.NaN;
You always refer to a property of the predefined Number
object as shown above, and not as a property of a Number
object you create yourself.
The following table summarizes the Number
object's properties.
Property | Description |
---|---|
MAX_VALUE |
The largest representable number |
MIN_VALUE |
The smallest representable number |
NaN |
Special "not a number" value |
NEGATIVE_INFINITY |
Special negative infinite value; returned on overflow |
POSITIVE_INFINITY |
Special positive infinite value; returned on overflow |
The Number prototype provides methods for retrieving information from Number objects in various formats. The following table summarizes the methods of Number.prototype
.
Method | Description |
---|---|
toExponential |
Returns a string representing the number in exponential notation. |
toFixed |
Returns a string representing the number in fixed-point notation. |
toPrecision |
Returns a string representing the number to a specified precision in fixed-point notation. |
toSource |
Returns an object literal representing the specified Number object; you can use this value to create a new object. Overrides the Object.toSource method. |
toString |
Returns a string representing the specified object. Overrides the Object.toString method. |
valueOf |
Returns the primitive value of the specified object. Overrides the Object.valueOf method. |
RegExp Object
The RegExp
object lets you work with regular expressions. It is described in Regular Expressions.
String Object
The String
object is a wrapper around the string primitive data type. Do not confuse a string literal with the String
object. For example, the following code creates the string literal s1
and also the String
object s2
:
var s1 = "foo"; //creates a string literal value var s2 = new String("foo"); //creates a String object
You can call any of the methods of the String
object on a string literal value—JavaScript automatically converts the string literal to a temporary String
object, calls the method, then discards the temporary String
object. You can also use the String.length
property with a string literal.
You should use string literals unless you specifically need to use a String
object, because String
objects can have counterintuitive behavior. For example:
var s1 = "2 + 2"; //creates a string literal value var s2 = new String("2 + 2"); //creates a String object eval(s1); //returns the number 4 eval(s2); //returns the string "2 + 2"
A String
object has one property, length
, that indicates the number of characters in the string. For example, the following code assigns x
the value 13, because "Hello, World!" has 13 characters:
var mystring = "Hello, World!"; var x = mystring.length;
A String
object has two types of methods: those that return a variation on the string itself, such as substring
and toUpperCase
, and those that return an HTML-formatted version of the string, such as bold
and link
.
For example, using the previous example, both mystring.toUpperCase()
and "hello, world!".toUpperCase()
return the string "HELLO, WORLD!"
The substring
method takes two arguments and returns a subset of the string between the two arguments. Using the previous example, mystring.substring(4, 9)
returns the string "o, Wo". See the substring
method of the String
object in the JavaScript Reference for more information.
The String
object also has a number of methods for automatic HTML formatting, such as bold
to create boldface text and link
to create a hyperlink. For example, you could create a hyperlink to a hypothetical URL with the link
method as follows:
mystring.link("https://www.helloworld.com")
The following table summarizes the methods of String
objects.
Method | Description |
---|---|
anchor |
Creates HTML named anchor. |
big , blink , bold , fixed , italics , small , strike , sub , sup |
Create HTML formatted string. |
charAt , charCodeAt |
Return the character or character code at the specified position in string. |
indexOf , lastIndexOf |
Return the position of specified substring in the string or last position of specified substring, respectively. |
link |
Creates HTML hyperlink. |
concat |
Combines the text of two strings and returns a new string. |
fromCharCode |
Constructs a string from the specified sequence of Unicode values. This is a method of the String class, not a String instance. |
split |
Splits a String object into an array of strings by separating the string into substrings. |
slice |
Extracts a section of an string and returns a new string. |
substring , substr |
Return the specified subset of the string, either by specifying the start and end indexes or the start index and a length. |
match , replace , search |
Work with regular expressions. |
toLowerCase , toUpperCase |
Return the string in all lowercase or all uppercase, respectively. |