Array

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De JavaScript Array klasse is een globaal object dat wordt gebruikt bij de constructie van arrays; een hoog-geplaatst, lijstachtig object.

Beschrijving

Arrays zijn lijstachtige objecten waarvan het prototype methoden heeft om te iterereren, muteren en kopiëren. Noch de lengte van een Javascript-array, noch de typen elementen staan vast. Aangezien de lengte van een array op elk moment kan veranderen en gegevens kunnen worden opgeslagen op niet-aangrenzende locaties, is het niet gegarandeerd dat de gegevensplekken in de Javascript-array vast staan. Over het algemeen zijn dit handige kenmerken; maar als deze functies niet wenselijk zijn voor jouw specifieke gebruik, kun je overwegen om Typed Arrays te gebruiken.

Arrays kunnen geen tekenreeksen gebruiken als elementindexen (zoals in een associatieve array), maar moeten hele getallen gebruiken. Een element instellen of ophalen met behulp van de haakjesnotatie (of puntnotatie) zal geen element uit de array ophalen of instellen. Maar zal proberen om een variabele uit de object property collection van de array op te halen of in te stellen. De objecteigenschappen van de array en de lijst met arrayelementen zijn namelijk gescheiden en de kopie- en mutatiebewerkingen van de array kunnen niet worden toegepast op deze benoemde eigenschappen

Alledaagse handelingen

Maak een Array aan

let fruit = ["Appel", "Banaan"];

console.log(fruit.length);
// 2

Toegang tot (indexeren van) een Array-item

let eerste = fruit[0];
// Appel

let laatste = fruit[fruit.length - 1];
// Banaan

Itereer over een Array

fruit.forEach(function (item, index, array) {
  console.log(item, index);
});
// Appel 0
// Banaan 1

Toevoegen op het eind van de Array

let nieuweLengte = fruit.push("Sinaasappel");
// ["Appel", "Banaan", "Sinaasappel"]

Verwijder op het eind van de Array

let laatste = fruit.pop(); // verwijder de Sinaasappel op het eind
// ["Appel", "Banaan"];

Verwijder van de eerste plaats van een array

let eerste = fruit.shift(); // verwijder appel van de eerste plaats
// ["Banaan"];

Voeg toe aan de eerste plaats van een Array

let nieuweLengte = fruit.unshift("Aardbei") // voeg de aarbei toe op de eerste plaats
// ["Aardbei", "Banaan"];

Zoek de index van een item in de Array

fruit.push("Mango");
// ["Aardbei", "Banaan", "Mango"]

let positie = fruit.indexOf("Banaan");
// 1

Verwijder een item van de indexpositie

let verwijderItem = fruit.splice(positie, 1); // hiermee kan je een item verwijderen
// ["Aardbei", "Mango"]

Kopieer een array

let Kopie = fruit.slice(); // hiermee maak je een kopie van de matrix
// ["Aardbei", "Mango"]

Syntaxis

[element0, element1, ..., elementN]
new Array(element0, element1[, ...[, elementN]])
new Array(arrayLength)
elementN
Een JavaScript-array wordt geïnitialiseerd met de gegeven elementen, behalve in het geval dat een enkel argument wordt doorgegeven aan de Array-constructor en dat argument een getal is. (Zie hieronder.) Merk op dat dit speciale geval alleen van toepassing is op JavaScript-arrays die zijn gemaakt met de Array-constructor, niet op array-literals die zijn gemaakt met de haakjesyntaxis.
arrayLengte
Als het enige argument dat aan de constructor Array is doorgegeven, een geheel getal tussen 0 en 232-1 (inclusief) is, geeft dit een nieuwe JavaScript-array terug waarvan de lengte is ingesteld op dat aantal. Als het argument een ander getal is, wordt er een uitzondering RangeError gegenereerd.

Toegang tot array-elementen

JavaScript-arrays zijn geïndexeerd vanaf nul: het eerste element van een array staat op index 0 en het laatste element staat op de index die gelijk is aan de waarde van de eigenschap length van de array min 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.
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.observe()
Asynchronously observes changes to Arrays, similar to Object.observe() for objects. It provides a stream of changes in order of occurrence.
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 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 ES6 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

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 (ECMA-262)
The definition of 'Array' in that specification.
Living Standard New method added: Array.prototype.includes()

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!
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)

See also