JavaScript Arrays
JavaScript arrays are used to store multiple values in a single variable.
Displaying Arrays
In this tutorial we will use a script to display arrays inside a <p> element with id="demo":
Example
<p id="demo"></p>
<script>
var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
</script>
The first line (in the script) creates an array named cars.
The second line "finds" the element with id="demo", and "displays" the array in the "innerHTML" of it.
Try it Yourself
Create an array, and assign values to it:
Spaces and line breaks are not important. A declaration can span multiple lines:
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Never put a comma after the last element (like "BMW",). The effect is inconsistent across browsers. |
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What is an Array?
An array is a special variable, which can hold more than one value at a time.
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";
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:
var array-name = [item1, item2, ...];
Example:
var cars = ["Saab", "Volvo", "BMW"];
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
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The two examples above do exactly the same. There is no need to use new
Array(). For simplicity, readability and execution speed, use the first one (the array literal method). |
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Access the Elements of an Array
You refer to an array element by referring to the index number.
This statement accesses the value of the first element in cars:
var
name = cars[0];
This statement modifies the first element in cars:
cars[0] = "Opel";
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[0] is the first element in an array. [1] is the second. Array indexes start with 0. |
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You Can Have Different Objects in One Array
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;
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:
Objects use names to access its "members". In this example, person.firstName returns John:
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 in cars
var y = cars.sort();
// The sort() method sort cars in alphabetical order
Array methods are covered in the next chapter.
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 »
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The length property is always one more than the highest array index. |
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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 »
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 »
Adding elements with high indexes can create undefined "holes" in an array:
Example
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[10] = "Lemon"; // adds a new element (Lemon) to fruits
Try it Yourself »
Looping Array Elements
The best way to loop through an array, is using a "for" loop:
Example
var index;
var fruits = ["Banana", "Orange", "Apple", "Mango"];
for (index = 0; index < fruits.length; index++) {
text += fruits[index];
}
Try it Yourself »
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 »
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WARNING !! If you use a named index, JavaScript will redefine the array to a standard object. After that, all array methods and properties will produce incorrect results. |
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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 »
The Difference Between Arrays and Objects
In JavaScript, arrays use numbered indexes.
In JavaScript, objects use named indexes.
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Arrays are a special kind of objects, with numbered indexes. |
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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
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 »
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 »
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():
Array.isArray(fruits); // returns true
Try it Yourself »
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 »
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
Try it Yourself »
Test Yourself with Exercises!
Exercise 1 » Exercise 2 » Exercise 3 » Exercise 4 » Exercise 5 »
