Tömbök, amelyek magas-szintű lista jellegű objektumok, létrehozásához használatos a JavaScript Array
objektum.
Leírás
A tömbök listaszerű objektumok amelyek prototípusa olyan metódusokat tartalmaz amelyekkel bejárhatóak és mutálhatóak. A JavaScipt tömbnek sem a hossza, sem az elemeinek típusa sem fix. A tömbön belül az adatok nem folytonos módon tárolhatóak, így mivel a tömb hossza bármikor megváltozhat a Javascript tömbök sürűsége nem garantált, ez a programozó által választott felhasználási módtól függ. Általánosságban ezek kényelmes tulajdonságok de ha ezek a jellemzők nem kívánatosak az ön számára érdemes lehet inkább típusos tömböket használni. A tömbök nem használhatnak stringeket elem indexként (mint egy asszociatív tömbben) csak kötelezően integereket. Ha a zárójel jelölés (vagy pont jelölés) segítségével nem-integert állítunk be vagy férünk hozzá akkor nem a tömb elemét fogjuk megkapni hanem a tömb objektum tulajdonság kollekciójának változóját. A tömb elemeinek listája és a tömb objektum tulajdonságai különböznek és a tömb bejáró és mutáló operátorait nem használhatjuk ezekhez az elnevezett tulajdonságokhoz.
Gyakori műveletek
Array létrehozása
var fruits = ["Apple", "Banana"];
console.log(fruits.length);
// 2
Egy Array elem elérése (indexelése)
var first = fruits[0];
// Apple
var last = fruits[fruits.length - 1];
// Banana
Array bejárása
fruits.forEach(function (item, index, array) {
console.log(item, index);
});
// Apple 0
// Banana 1
Hozzáadás egy Array végéhez
var newLength = fruits.push("Orange");
// ["Apple", "Banana", "Orange"]
Array végéről elem eltávolítása
var last = fruits.pop(); // Orange eltávolítása (a végéről)
// ["Apple", "Banana"];
Array elejéről elem eltávolítása
var first = fruits.shift(); // eltávolítja az Apple elemet az elejéről
// ["Banana"];
Array elejéhez hozzáadás
var newLength = fruits.unshift("Strawberry") // hozzáadás az elejéhez
// ["Strawberry", "Banana"];
Array elem indexének megkeresése
fruits.push("Mango");
// ["Strawberry", "Banana", "Mango"]
var pos = fruits.indexOf("Banana");
// 1
Index pozició alapján elem eltávolítása
var removedItem = fruits.splice(pos, 1); // így távolítunk el egy elemet
// ["Strawberry", "Mango"]
Index pozició alapján elemek eltávolítása
let vegetables = ['Cabbage', 'Turnip', 'Radish', 'Carrot']
console.log(vegetables)
// ["Cabbage", "Turnip", "Radish", "Carrot"]
let pos = 1
let n = 2
let removedItems = vegetables.splice(pos, n)
// this is how to remove items, n defines the number of items to be removed,
// from that position(pos) onward to the end of array.
console.log(vegetables)
// ["Cabbage", "Carrot"] (the original array is changed)
console.log(removedItems)
// ["Turnip", "Radish"]
Array másolása
var shallowCopy = fruits.slice(); // this is how to make a copy
// ["Strawberry"]
Syntax
[element0, element1, ..., elementN]
new Array(element0, element1[, ...[, elementN]])
new Array(arrayLength)
Parameters
elementN
- A JavaScript array is initialized with the given elements, except in the case where a single argument is passed to the
Array
constructor and that argument is a number (see the arrayLength parameter below). Note that this special case only applies to JavaScript arrays created with theArray
constructor, not array literals created with the bracket syntax. arrayLength
- If the only argument passed to the
Array
constructor is an integer between 0 and 232-1 (inclusive), this returns a new JavaScript array with length set to that number. If the argument is any other number, aRangeError
exception is thrown.
Description
Arrays are list-like objects whose prototype has methods to perform traversal and mutation operations. Neither the length of a JavaScript array nor the types of its elements are fixed. Since an array's length can change at any time, and data can be stored at non-contiguous locations in the array, JavaScript arrays are not guaranteed to be dense; this depends on how the programmer chooses to use them. In general, these are convenient characteristics; but if these features are not desirable for your particular use, you might consider using typed arrays.
Some people think that you shouldn't use an array as an associative array. In any case, you can use plain objects
instead, although doing so comes with its own caveats. See the post Lightweight JavaScript dictionaries with arbitrary keys as an example.
Accessing array elements
JavaScript arrays are zero-indexed: the first element of an array is at index 0
, and the last element is at the index equal to the value of the array's length
property minus 1.
var arr = ['this is the first element', 'this is the second element'];
console.log(arr[0]); // logs 'this is the first element'
console.log(arr[1]); // logs 'this is the second element'
console.log(arr[arr.length - 1]); // logs 'this is the second element'
Array elements are object properties in the same way that toString
is a property, but trying to access an element of an array as follows throws a syntax error, because the property name is not valid:
console.log(arr.0); // a syntax error
There is nothing special about JavaScript arrays and the properties that cause this. JavaScript properties that begin with a digit cannot be referenced with dot notation; and must be accessed using bracket notation. For example, if you had an object with a property named '3d'
, it can only be referenced using bracket notation. E.g.:
var years = [1950, 1960, 1970, 1980, 1990, 2000, 2010];
console.log(years.0); // a syntax error
console.log(years[0]); // works properly
renderer.3d.setTexture(model, 'character.png'); // a syntax error
renderer['3d'].setTexture(model, 'character.png'); // works properly
Note that in the 3d
example, '3d'
had to be quoted. It's possible to quote the JavaScript array indexes as well (e.g., years['2']
instead of years[2]
), although it's not necessary. The 2 in years[2]
is coerced into a string by the JavaScript engine through an implicit toString
conversion. It is for this reason that '2'
and '02'
would refer to two different slots on the years
object and the following example could be true
:
console.log(years['2'] != years['02']);
Similarly, object properties which happen to be reserved words(!) can only be accessed as string literals in bracket notation(but it can be accessed by dot notation in firefox 40.0a2 at least):
var promise = {
'var' : 'text',
'array': [1, 2, 3, 4]
};
console.log(promise['array']);
Relationship between length and numerical properties
A JavaScript array's length
property and numerical properties are connected. Several of the built-in array methods (e.g., join
, slice
, indexOf
, etc.) take into account the value of an array's length
property when they're called. Other methods (e.g., push
, splice
, etc.) also result in updates to an array's length
property.
var fruits = [];
fruits.push('banana', 'apple', 'peach');
console.log(fruits.length); // 3
When setting a property on a JavaScript array when the property is a valid array index and that index is outside the current bounds of the array, the engine will update the array's length
property accordingly:
fruits[5] = 'mango';
console.log(fruits[5]); // 'mango'
console.log(Object.keys(fruits)); // ['0', '1', '2', '5']
console.log(fruits.length); // 6
Increasing the length
.
fruits.length = 10;
console.log(Object.keys(fruits)); // ['0', '1', '2', '5']
console.log(fruits.length); // 10
Decreasing the length
property does, however, delete elements.
fruits.length = 2;
console.log(Object.keys(fruits)); // ['0', '1']
console.log(fruits.length); // 2
This is explained further on the Array.length
page.
Creating an array using the result of a match
The result of a match between a regular expression and a string can create a JavaScript array. This array has properties and elements which provide information about the match. Such an array is returned by RegExp.exec
, String.match
, and String.replace
. To help explain these properties and elements, look at the following example and then refer to the table below:
// Match one d followed by one or more b's followed by one d
// Remember matched b's and the following d
// Ignore case
var myRe = /d(b+)(d)/i;
var myArray = myRe.exec('cdbBdbsbz');
The properties and elements returned from this match are as follows:
Property/Element | Description | Example |
input |
A read-only property that reflects the original string against which the regular expression was matched. | cdbBdbsbz |
index |
A read-only property that is the zero-based index of the match in the string. | 1 |
[0] |
A read-only element that specifies the last matched characters. | dbBd |
[1], ...[n] |
Read-only elements that specify the parenthesized substring matches, if included in the regular expression. The number of possible parenthesized substrings is unlimited. | [1]: bB [2]: d |
Properties
Array.length
- The
Array
constructor's length property whose value is 1. get Array[@@species]
- The constructor function that is used to create derived objects.
Array.prototype
- Allows the addition of properties to all array objects.
Methods
Array.from()
- Creates a new
Array
instance from an array-like or iterable object. Array.isArray()
- Returns true if a variable is an array, if not false.
Array.of()
- Creates a new
Array
instance with a variable number of arguments, regardless of number or type of the arguments.
Array instances
All Array
instances inherit from Array.prototype
. The prototype object of the Array
constructor can be modified to affect all Array
instances.
Properties
Methods
Mutator methods
Accessor methods
Iteration methods
Array generic methods
Array generics are non-standard, deprecated and will get removed in the near future. Note that you can not rely on them cross-browser. However, there is a shim available on GitHub.
Sometimes you would like to apply array methods to strings or other array-like objects (such as function arguments). By doing this, you treat a string as an array of characters (or otherwise treat a non-array as an array). For example, in order to check that every character in the variable str is a letter, you would write:
function isLetter(character) {
return character >= 'a' && character <= 'z';
}
if (Array.prototype.every.call(str, isLetter)) {
console.log("The string '" + str + "' contains only letters!");
}
This notation is rather wasteful and JavaScript 1.6 introduced a generic shorthand:
if (Array.every(str, isLetter)) {
console.log("The string '" + str + "' contains only letters!");
}
Generics are also available on String
.
These are not part of ECMAScript standards (though the ES2015 Array.from()
can be used to achieve this). The following is a shim to allow its use in all browsers:
// Assumes Array extras already present (one may use polyfills for these as well)
(function() {
'use strict';
var i,
// We could also build the array of methods with the following, but the
// getOwnPropertyNames() method is non-shimable:
// Object.getOwnPropertyNames(Array).filter(function(methodName) {
// return typeof Array[methodName] === 'function'
// });
methods = [
'join', 'reverse', 'sort', 'push', 'pop', 'shift', 'unshift',
'splice', 'concat', 'slice', 'indexOf', 'lastIndexOf',
'forEach', 'map', 'reduce', 'reduceRight', 'filter',
'some', 'every', 'find', 'findIndex', 'entries', 'keys',
'values', 'copyWithin', 'includes'
],
methodCount = methods.length,
assignArrayGeneric = function(methodName) {
if (!Array[methodName]) {
var method = Array.prototype[methodName];
if (typeof method === 'function') {
Array[methodName] = function() {
return method.call.apply(method, arguments);
};
}
}
};
for (i = 0; i < methodCount; i++) {
assignArrayGeneric(methods[i]);
}
}());
Examples
Creating an array
The following example creates an array, msgArray
, with a length of 0, then assigns values to msgArray[0]
and msgArray[99]
, changing the length of the array to 100.
var msgArray = [];
msgArray[0] = 'Hello';
msgArray[99] = 'world';
if (msgArray.length === 100) {
console.log('The length is 100.');
}
Creating a two-dimensional array
The following creates a chess board as a two dimensional array of strings. The first move is made by copying the 'p' in (6,4) to (4,4). The old position (6,4) is made blank.
var board = [
['R','N','B','Q','K','B','N','R'],
['P','P','P','P','P','P','P','P'],
[' ',' ',' ',' ',' ',' ',' ',' '],
[' ',' ',' ',' ',' ',' ',' ',' '],
[' ',' ',' ',' ',' ',' ',' ',' '],
[' ',' ',' ',' ',' ',' ',' ',' '],
['p','p','p','p','p','p','p','p'],
['r','n','b','q','k','b','n','r'] ];
console.log(board.join('\n') + '\n\n');
// Move King's Pawn forward 2
board[4][4] = board[6][4];
board[6][4] = ' ';
console.log(board.join('\n'));
Here is the output:
R,N,B,Q,K,B,N,R P,P,P,P,P,P,P,P , , , , , , , , , , , , , , , , , , , , , , , , , , , , p,p,p,p,p,p,p,p r,n,b,q,k,b,n,r R,N,B,Q,K,B,N,R P,P,P,P,P,P,P,P , , , , , , , , , , , , , , , , , ,p, , , , , , , , , , p,p,p,p, ,p,p,p r,n,b,q,k,b,n,r
Using an array to tabulate a set of values
values=[];
for (x=0; x<10; x++){
values.push([
2**x,
2*x**2
])
};
console.table(values)
Results in
0 1 0 1 2 2 2 4 8 3 8 18 4 16 32 5 32 50 6 64 72 7 128 98 8 256 128 9 512 162
(First column is the (index))
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 1st Edition (ECMA-262) | Standard | Initial definition. |
ECMAScript 5.1 (ECMA-262) The definition of 'Array' in that specification. |
Standard | New methods added: Array.isArray , indexOf , lastIndexOf , every , some , forEach , map , filter , reduce , reduceRight |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array' in that specification. |
Standard | New methods added: Array.from , Array.of , find , findIndex , fill , copyWithin |
ECMAScript Latest Draft (ECMA-262) The definition of 'Array' in that specification. |
Draft | New method added: Array.prototype.includes() |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |