JavaScript arrays are used to store multiple values in a single variable.
Example
Try it Yourself »What is an Array?
- Use Object.entries(obj) to get an array of key/value pairs from obj. Use array methods on that array, e.g. Use Object.fromEntries(array) on the resulting array to turn it back into an object. For example, we have an object with prices, and would like to double them.
- Iterable objects (objects such as Map and Set). Array.from has an optional parameter mapFn, which allows you to execute a map function on each element of the array being created. More clearly, Array.from(obj, mapFn, thisArg) has the same result as Array.from(obj).map(mapFn, thisArg), except that it does not create an intermediate array.
An array is a special variable, which can hold more than one value at a time.
Jul 20, 2013 Sequences using JavaScript Array. Jul 20, 2013 5 min read #es6 #javascript #tip. Generating a sequence is a common programming task. This is rather easy to achieve using a straightforward loop. With JavaScript however, there exists a more functional variant using the powerful Array object.
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:
var car1 = 'Saab';
var car2 = 'Volvo';
var car3 = 'BMW';
var car2 = 'Volvo';
var car3 = 'BMW';
However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?
The solution is an array!
An array can hold many values under a single name, and you can access the values by referring to an index number.
Creating an Array
Using an array literal is the easiest way to create a JavaScript Array.
Syntax:
Example
Try it Yourself »Spaces and line breaks are not important. A declaration can span multiple lines:
Example
Try it Yourself »Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
Example
Try it Yourself »The two examples above do exactly the same. There is no need to use
For simplicity, readability and execution speed, use the first one (the array literal method).
new Array()
.For simplicity, readability and execution speed, use the first one (the array literal method).
Access the Elements of an Array
You access an array element by referring to the index number.
This statement accesses the value of the first element in
cars
:Example
var cars = ['Saab', 'Volvo', 'BMW'];
document.getElementById('demo').innerHTML = cars[0];
Try it Yourself »document.getElementById('demo').innerHTML = cars[0];
Note: Array indexes start with 0.
[0] is the first element. [1] is the second element.
Changing an Array Element
This statement changes the value of the first element in
cars
:Example
var cars = ['Saab', 'Volvo', 'BMW'];
cars[0] = 'Opel';
document.getElementById('demo').innerHTML = cars[0];
Try it Yourself »cars[0] = 'Opel';
document.getElementById('demo').innerHTML = cars[0];
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:
Example
var cars = ['Saab', 'Volvo', 'BMW'];
document.getElementById('demo').innerHTML = cars;
Try it Yourself »document.getElementById('demo').innerHTML = cars;
Arrays are Objects
Arrays are a special type of objects. The
typeof
operator in JavaScript returns 'object' for arrays.But, JavaScript arrays are best described as arrays.
Arrays use numbers to access its 'elements'. In this example,
person[0]
returns John:Array:
Try it Yourself »Objects use names to access its 'members'. In this example,
person.firstName
returns John:Object:
var person = {firstName:'John', lastName:'Doe', age:46};
Try it Yourself »Array Elements Can Be Objects
JavaScript variables can be objects. Arrays are special kinds of objects.
Because of this, you can have variables of different types in the same Array.
You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:
myArray[0] = Date.now;
myArray[1] = myFunction;
myArray[2] = myCars;
myArray[1] = myFunction;
myArray[2] = myCars;
Array Properties and Methods
The real strength of JavaScript arrays are the built-in array properties and methods:
Examples
var x = cars.length; // The length property returns the number of elements
var y = cars.sort(); // The sort() method sorts arrays
var y = cars.sort(); // The sort() method sorts arrays
Array methods are covered in the next chapters.
The length Property
The
length
property of an array returns the length of an array (the number of array elements).Example
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits.length; // the length of fruits is 4
Try it Yourself »fruits.length; // the length of fruits is 4
The
length
property is always one more than the highest array index.Accessing the First Array Element
Example
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
var first = fruits[0];
Try it Yourself »var first = fruits[0];
Accessing the Last Array Element
Example
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
var last = fruits[fruits.length - 1];
Try it Yourself »var last = fruits[fruits.length - 1];
Looping Array Elements
The safest way to loop through an array, is using a
for
loop:Example
var fruits, text, fLen, i;
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fLen = fruits.length;
text = '<ul>';
for (i = 0; i < fLen; i++) {
text += '<li>' + fruits[i] + '</li>';
}
text += '</ul>';
Try it Yourself »fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fLen = fruits.length;
text = '<ul>';
for (i = 0; i < fLen; i++) {
text += '<li>' + fruits[i] + '</li>';
}
text += '</ul>';
You can also use the
Array.forEach()
function:Example
var fruits, text;
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
text = '<ul>';
fruits.forEach(myFunction);
text += '</ul>';
function myFunction(value) {
text += '<li>' + value + '</li>';
}
Try it Yourself »fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
text = '<ul>';
fruits.forEach(myFunction);
text += '</ul>';
function myFunction(value) {
text += '<li>' + value + '</li>';
}
Adding Array Elements
The easiest way to add a new element to an array is using the
push()
method:Example
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits.push('Lemon'); // adds a new element (Lemon) to fruits
Try it Yourself »fruits.push('Lemon'); // adds a new element (Lemon) to fruits
New element can also be added to an array using the
length
property:Example
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits[fruits.length] = 'Lemon'; // adds a new element (Lemon) to fruits
Try it Yourself »fruits[fruits.length] = 'Lemon'; // adds a new element (Lemon) to fruits
WARNING !
Adding elements with high indexes can create undefined 'holes' in an array:
Example
var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits[6] = 'Lemon'; // adds a new element (Lemon) to fruits
Try it Yourself »fruits[6] = 'Lemon'; // adds a new element (Lemon) to fruits
Associative Arrays
Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative arrays (or hashes).
JavaScript does not support arrays with named indexes.
In JavaScript, arrays always use numbered indexes.
Example
var person = [];
person[0] = 'John';
person[1] = 'Doe';
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return 'John'
Try it Yourself »person[0] = 'John';
person[1] = 'Doe';
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return 'John'
WARNING !!
If you use named indexes, JavaScript will redefine the array to a standard object.
After that, some array methods and properties will produce incorrect results.
If you use named indexes, JavaScript will redefine the array to a standard object.
After that, some array methods and properties will produce incorrect results.
Example:
var person = [];
person['firstName'] = 'John';
person['lastName'] = 'Doe';
person['age'] = 46;
var x = person.length; // person.length will return 0
var y = person[0]; // person[0] will return undefined
Try it Yourself »person['firstName'] = 'John';
person['lastName'] = 'Doe';
person['age'] = 46;
var x = person.length; // person.length will return 0
var y = person[0]; // person[0] will return undefined
The Difference Between Arrays and Objects
In JavaScript, arrays use numbered indexes.
In JavaScript, objects use named indexes.
Arrays are a special kind of objects, with numbered indexes.
When to Use Arrays. When to use Objects.
- JavaScript does not support associative arrays.
- You should use objects when you want the element names to be strings (text).
- You should use arrays when you want the element names to be numbers.
Avoid new Array()
There is no need to use the JavaScript's built-in array constructor
new
Array().Use
[]
instead.These two different statements both create a new empty array named points:
var points = new Array(); // Bad
var points = []; // Good
var points = []; // Good
These two different statements both create a new array containing 6 numbers:
var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
Try it Yourself »var points = [40, 100, 1, 5, 25, 10]; // Good
The
new
keyword only complicates the code. It can also produce some unexpected results: var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
What if I remove one of the elements?
var points = new Array(40); // Creates an array with 40 undefined elements !!!!!
Try it Yourself »How to Recognize an Array
A common question is: How do I know if a variable is an array?
The problem is that the JavaScript operator
typeof
returns 'object
': var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
typeof fruits; // returns object
Try it Yourself »typeof fruits; // returns object
The typeof operator returns object because a JavaScript array is an object.
Solution 1:
To solve this problem ECMAScript 5 defines a new method
Array.isArray()
:The problem with this solution is that ECMAScript 5 is not supported in older browsers.
Solution 2:
To solve this problem you can create your own
isArray()
function: function isArray(x) {
return x.constructor.toString().indexOf('Array') > -1;
}
Try it Yourself »return x.constructor.toString().indexOf('Array') > -1;
}
The function above always returns true if the argument is an array.
Or more precisely: it returns true if the object prototype contains the word 'Array'.
Solution 3:
The
instanceof
operator returns true if an object is created by a given constructor: var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits instanceof Array; // returns true
fruits instanceof Array; // returns true
Creating An Array In Javascript
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