Array

In JavaScript, arrays aren't primitives but are instead Array objects with the following core characteristics:

• JavaScript arrays are resizable and can contain a mix of different data types. (When those characteristics are undesirable, use typed arrays instead.)
• JavaScript arrays are not associative arrays and so, array elements cannot be accessed using strings as indexes, but must be accessed using integers as indexes.
• JavaScript arrays are zero-indexed: the first element of an array is at index 0, the second is at index 1, and so on — and the last element is at the value of the array's length property minus 1.
• JavaScript array-copy operations create shallow copies. (All standard built-in copy operations with any JavaScript objects create shallow copies, rather than deep copies).

Array()

Creates a new Array object.

get Array[@@species]

Returns the Array constructor.

Array.from()

Creates a new Array instance from an array-like object or iterable object.

Array.isArray()

Returns true if the argument is an array, or false otherwise.

Array.of()

Creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.

Array.prototype.length

Reflects the number of elements in an array.

Array.prototype[@@unscopables]

Contains property names that were not included in the ECMAScript standard prior to the ES2015 version and that are ignored for with statement-binding purposes.

Array.prototype.at()

Returns the array item at the given index. Accepts negative integers, which count back from the last item.

Array.prototype.concat()

Returns a new array that is the calling array joined with other array(s) and/or value(s).

Array.prototype.copyWithin()

Copies a sequence of array elements within an array.

Array.prototype.entries()

Returns a new array iterator object that contains the key/value pairs for each index in an array.

Array.prototype.every()

Returns true if every element in the calling array satisfies the testing function.

Array.prototype.fill()

Fills all the elements of an array from a start index to an end index with a static value.

Array.prototype.filter()

Returns a new array containing all elements of the calling array for which the provided filtering function returns true.

Array.prototype.find()

Returns the found element in the calling array, if some element in the array satisfies the testing function, or undefined if not found.

Array.prototype.findIndex()

Returns the found index in the calling array, if an element in the array satisfies the testing function, or -1 if not found.

Array.prototype.flat()

Returns a new array with all sub-array elements concatenated into it recursively up to the specified depth.

Array.prototype.flatMap()

Returns a new array formed by applying a given callback function to each element of the calling array, and then flattening the result by one level.

Array.prototype.forEach()

Calls a function for each element in the calling array.

Array.prototype.groupBy() Experimental

Groups the elements of an array into an object according to the strings returned by a test function.

Array.prototype.groupByToMap() Experimental

Groups the elements of an array into a Map according to values returned by a test function.

Array.prototype.includes()

Determines whether the calling array contains a value, returning true or false as appropriate.

Array.prototype.indexOf()

Returns the first (least) index at which a given element can be found in the calling array.

Array.prototype.join()

Joins all elements of an array into a string.

Array.prototype.keys()

Returns a new array iterator that contains the keys for each index in the calling array.

Array.prototype.lastIndexOf()

Returns the last (greatest) index at which a given element can be found in the calling array, or -1 if none is found.

Array.prototype.map()

Returns a new array containing the results of invoking a function on every element in the calling array.

Array.prototype.pop()

Removes the last element from an array and returns that element.

Array.prototype.push()

Adds one or more elements to the end of an array, and returns the new length of the array.

Array.prototype.reduce()

Executes a user-supplied "reducer" callback function on each element of the array (from left to right), to reduce it to a single value.

Array.prototype.reduceRight()

Executes a user-supplied "reducer" callback function on each element of the array (from right to left), to reduce it to a single value.

Array.prototype.reverse()

Reverses the order of the elements of an array in place. (First becomes the last, last becomes first.)

Array.prototype.shift()

Removes the first element from an array and returns that element.

Array.prototype.slice()

Extracts a section of the calling array and returns a new array.

Array.prototype.some()

Returns true if at least one element in the calling array satisfies the provided testing function.

Array.prototype.sort()

Sorts the elements of an array in place and returns the array.

Array.prototype.splice()

Adds and/or removes elements from an array.

Array.prototype.toLocaleString()

Returns a localized string representing the calling array and its elements. Overrides the Object.prototype.toLocaleString() method.

Array.prototype.toString()

Returns a string representing the calling array and its elements. Overrides the Object.prototype.toString() method.

Array.prototype.unshift()

Adds one or more elements to the front of an array, and returns the new length of the array.

Array.prototype.values()

Returns a new array iterator object that contains the values for each index in the array.

Array.prototype[@@iterator]()

Returns the values() function by default.

This section provides some examples of common array operations in JavaScript.

This example shows three ways to create new array: first using array literal notation, then using the Array() constructor, and finally using String.prototype.split() to build the array from a string.

// 'fruits' array created using array literal notation.
const fruits = ['Apple', 'Banana'];
console.log(fruits.length);
// 2

// 'fruits' array created using the Array() constructor.
const fruits = new Array('Apple', 'Banana');
console.log(fruits.length);
// 2

// 'fruits' array created using String.prototype.split().
const fruits = 'Apple, Banana'.split(', ');
console.log(fruits.length);
// 2

This example uses the join() method to create a string from the fruits array.

const fruits = ['Apple', 'Banana'];
const fruitsString = fruits.join(', ');
console.log(fruitsString);
// "Apple, Banana"

This example shows how to access items in the fruits array by specifying the index number of their position in the array.

const fruits = ['Apple', 'Banana'];

// The index of an array's first element is always 0.
fruits[0]; // Apple

// The index of an array's second element is always 1.
fruits[1]; // Banana

// The index of an array's last element is always one
// less than the length of the array.
fruits[fruits.length - 1]; // Banana

// Using a index number larger than the array's length
// returns 'undefined'.
fruits[99]; // undefined

This example uses the indexOf() method to find the position (index) of the string "Banana" in the fruits array.

const fruits = ['Apple', 'Banana'];
console.log(fruits.indexOf('Banana'));
// 1

This example shows two ways to check if the fruits array contains "Banana" and "Cherry": first with the includes() method, and then with the indexOf() method to test for an index value that's not -1.

const fruits = ['Apple', 'Banana'];

fruits.includes('Banana'); // true
fruits.includes('Cherry'); // false

// If indexOf() doesn't return -1, the array contains the given item.
fruits.indexOf('Banana') !== -1; // true
fruits.indexOf('Cherry') !== -1; // false

This example uses the push() method to append a new string to the fruits array.

const fruits = ['Apple', 'Banana'];
const newLength = fruits.push('Orange');
console.log(fruits);
// ["Apple", "Banana", "Orange"]
console.log(newLength);
// 3

This example uses the pop() method to remove the last item from the fruits array.

const fruits = ['Apple', 'Banana', 'Orange'];
const removedItem = fruits.pop();
console.log(fruits);
// ["Apple", "Banana"]
console.log(removedItem);
// Orange

This example uses the splice() method to remove the last 3 items from the fruits array.

const fruits = ['Apple', 'Banana', 'Strawberry', 'Mango', 'Cherry'];
const start = -3;
const removedItems = fruits.splice(start);
console.log(fruits);
// ["Apple", "Banana"]
console.log(removedItems);
// ["Strawberry", "Mango", "Cherry"]

This example uses the splice() method to truncate the fruits array down to just its first 2 items.

const fruits = ['Apple', 'Banana', 'Strawberry', 'Mango', 'Cherry'];
const start = 2;
const removedItems = fruits.splice(start);
console.log(fruits);
// ["Apple", "Banana"]
console.log(removedItems);
// ["Strawberry", "Mango", "Cherry"]

This example uses the shift() method to remove the first item from the fruits array.

const fruits = ['Apple', 'Banana'];
const removedItem = fruits.shift();
console.log(fruits);
// ["Banana"]
console.log(removedItem);
// Apple

This example uses the splice() method to remove the first 3 items from the fruits array.

const fruits = ['Apple', 'Strawberry', 'Cherry', 'Banana', 'Mango'];
const start = 0;
const deleteCount = 3;
const removedItems = fruits.splice(start, deleteCount);
console.log(fruits);
// ["Banana", "Mango"]
console.log(removedItems);
// ["Apple", "Strawberry", "Cherry"]

This example uses the unshift() method to add, at index 0, a new item to the fruits array — making it the new first item in the array.

const fruits = ['Banana', 'Mango'];
const newLength = fruits.unshift('Strawberry');
console.log(fruits);
// ["Strawberry", "Banana", "Mango"]
console.log(newLength);
// 3

This example uses the splice() method to remove the string "Banana" from the fruits array — by specifying the index position of "Banana".

const fruits = ['Strawberry', 'Banana', 'Mango'];
const start = fruits.indexOf('Banana');
const deleteCount = 1;
const removedItems = fruits.splice(start, deleteCount);
console.log(fruits);
// ["Strawberry", "Mango"]
console.log(removedItems);
// ["Banana"]

This example uses the splice() method to remove the strings "Banana" and "Strawberry" from the fruits array — by specifying the index position of "Banana", along with a count of the number of total items to remove.

const fruits = ['Apple', 'Banana', 'Strawberry', 'Mango'];
const start = 1;
const deleteCount = 2;
const removedItems = fruits.splice(start, deleteCount);
console.log(fruits);
// ["Apple", "Mango"]
console.log(removedItems);
// ["Banana", "Strawberry"]

This example uses the splice() method to replace the last 2 items in the fruits array with new items.

const fruits = ['Apple', 'Banana', 'Strawberry'];
const start = -2;
const deleteCount = 2;
const removedItems = fruits.splice(start, deleteCount, 'Mango', 'Cherry');
console.log(fruits);
// ["Apple", "Mango", "Cherry"]
console.log(removedItems);
// ["Banana", "Strawberry"]

This example uses a for...of loop to iterate over the fruits array, logging each item to the console.

const fruits = ['Apple', 'Mango', 'Cherry'];
for (const fruit of fruits) {
  console.log(fruit);
}
// Apple
// Mango
// Cherry

But for...of is just one of many ways to iterate over any array; for more ways, see Loops and iteration, and see the documentation for the every(), filter(), flatMap(), map(), reduce(), and reduceRight() methods — and see the next example, which uses the forEach() method.

This example uses the forEach() method to call a function on each element in the fruits array; the function causes each item to be logged to the console, along with the item's index number.

const fruits = ['Apple', 'Mango', 'Cherry'];
fruits.forEach(function(item, index, array) {
  console.log(item, index);
});
// Apple 0
// Mango 1
// Cherry 2

This example uses the concat() method to merge the fruits array with a moreFruits array, to produce a new combinedFruits array. Notice that fruits and moreFruits remain unchanged.

const fruits = ['Apple', 'Banana', 'Strawberry'];
const moreFruits = ['Mango', 'Cherry'];
const combinedFruits = fruits.concat(moreFruits);
console.log(combinedFruits);
// ["Apple", "Banana", "Strawberry", "Mango", "Cherry"]

// The 'fruits' array remains unchanged.
console.log(fruits);
// ["Apple", "Banana", "Strawberry"]

// The 'moreFruits' array also remains unchanged.
console.log(moreFruits);
// ["Mango", "Cherry"]

This example shows three ways to create a new array from the existing fruits array: first by using spread syntax, then by using the from() method, and then by using the slice() method.

const fruits = ['Strawberry', 'Mango'];

// Create a copy using spread syntax.
const fruitsCopy = [...fruits];
// ["Strawberry", "Mango"]

// Create a copy using the from() method.
const fruitsCopy = Array.from(fruits);
// ["Strawberry", "Mango"]

// Create a copy using the slice() method.
const fruitsCopy = fruits.slice();
// ["Strawberry", "Mango"]

All built-in array-copy operations (spread syntax, Array.from(), Array.prototype.slice(), and Array.prototype.concat()) create shallow copies. If you instead want a deep copy of an array, you can use JSON.stringify() to convert the array to a JSON string, and then JSON.parse() to convert the string back into a new array that's completely independent from the original array.

const fruitsDeepCopy = JSON.parse(JSON.stringify(fruits));

You can also create deep copies using the structuredClone() method, which has the advantage of allowing transferable objects in the source to be transferred to the new copy, rather than just cloned.

Finally, it's important to understand that assigning an existing array to a new variable doesn't create a copy of either the array or its elements. Instead the new variable is just a reference, or alias, to the original array; that is, the original array's name and the new variable name are just two names for the exact same object (and so will always evaluate as strictly equivalent). Therefore, if you make any changes at all either to the value of the original array or to the value of the new variable, the other will change, too:

const fruits = ['Strawberry', 'Mango'];
const fruitsAlias = fruits;
// 'fruits' and 'fruitsAlias' are the same object, strictly equivalent.
fruits === fruitsAlias // true
// Any changes to the 'fruits' array change 'fruitsAlias' too.
fruits.unshift('Apple', 'Banana');
console.log(fruits);
// ['Apple', 'Banana', 'Strawberry', 'Mango']
console.log(fruitsAlias);
// ['Apple', 'Banana', 'Strawberry', 'Mango']

The Array.prototype.groupBy() methods can be used to group the elements of an array, using a test function that returns a string indicating the group of the current element.

Here we have a simple inventory array that contains "food" objects that have a name and a type.

const inventory = [
  { name: 'asparagus', type: 'vegetables' },
  { name: 'bananas',  type: 'fruit' },
  { name: 'goat', type: 'meat' },
  { name: 'cherries', type: 'fruit' },
  { name: 'fish', type: 'meat' }
];

To use groupBy(), you supply a callback function that is called with the current element, and optionally the current index and array, and returns a string indicating the group of the element.

The code below uses a arrow function to return the type of each array element (this uses object destructuring syntax for function arguments to unpack the type element from the passed object). The result is an object that has properties named after the unique strings returned by the callback. Each property is assigned an array containing the elements in the group.

let result = inventory.groupBy( ({ type }) => type );
console.log(result.vegetables)
// expected output: Array [Object { name: "asparagus", type: "vegetables" }]

Note that the returned object references the same elements as the original array (not deep copies). Changing the internal structure of these elements will be reflected in both the original array and the returned object.

If you can't use a string as the key, for example, if the information to group is associated with an object that might change, then you can instead use Array.prototype.groupByToMap(). This is very similar to groupBy except that it groups the elements of the array into a Map that can use an arbitrary value (object or primitive) as a key.

The following creates a chessboard as a two-dimensional array of strings. The first move is made by copying the 'p' in board[6][4] to board[4][4]. The old position at [6][4] is made blank.

const 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

const values = [];
for (let x = 0; x < 10; x++) {
  values.push([
    2 ** x,
    2 * x ** 2,
  ]);
}
console.table(values);

Results in

// The first column is the index
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

Array objects cannot use strings as element indexes (as in an associative array) but must use integers. Setting or accessing via non-integers using bracket notation (or dot notation) will not set or retrieve an element from the array list itself, but will set or access a variable associated with that array's object property collection. The array's object properties and list of array elements are separate, and the array's traversal and mutation operations cannot be applied to these named properties.

Array elements are object properties in the same way that toString is a property (to be specific, however, toString() is a method). Nevertheless, 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.

const 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

In the 3d example, '3d' had to be quoted (because it begins with a digit). But it's also possible to quote the 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. As a result, '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']);

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.

const 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(fruits);              // ['banana', 'apple', 'peach', empty x 2, 'mango', empty x 4]
console.log(Object.keys(fruits)); // ['0', '1', '2', '5']
console.log(fruits.length);       // 10
console.log(fruits[8]);           // undefined

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.

The result of a match between a RegExp and a string can create a JavaScript array that has properties and elements which provide information about the match. Such an array is returned by RegExp.exec() and String.match().

To help explain these properties and elements, see 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

const myRe = /d(b+)(d)/i;
const myArray = myRe.exec('cdbBdbsbz');

The properties and elements returned from this match are as follows:

• From the JavaScript Guide:
  • "Indexing object properties"
  • "Indexed collections: Array object"
• Typed Arrays
• RangeError: invalid array length