JavaScript Arrays
JavaScript arrays are used to store multiple values in a single variable.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
var cars = [
"Saab",
"Volvo",
"BMW"
];
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
Putting a comma after the last element (like "BMW",) is inconsistent across browsers.
IE 8 and earlier will fail.
IE 8 and earlier will fail.
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
var cars = new Array("Saab", "Volvo", "BMW");
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
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).
For simplicity, readability and execution speed, use the first one (the array literal method).
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:
varname = cars[0];
This statement modifies the first element in cars:
cars[0] = "Opel";
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>JavaScript array elements are accesses using numeric indexes (starting from 0).</p>
<p id="demo"></p>
<script>
var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars[0];
</script>
</body>
</html>
[0] is the first element in an array. [1] is the second. Array indexes start with 0.
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
var cars = ["Saab", "Volvo", "BMW"];
document.getElementById("demo").innerHTML = cars;
</script>
</body>
</html>
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:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Arrays use numbers to access its elements.</p>
<p id="demo"></p>
<script>
var person = ["John", "Doe", 46];
document.getElementById("demo").innerHTML = person[0];
</script>
</body>
</html>
Object:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Objects</h2>
<p>JavaScript uses names to access object properties.</p>
<p id="demo"></p>
<script>
var person = {firstName:"John", lastName:"Doe", age:46};
document.getElementById("demo").innerHTML = person["firstName"];
</script>
</body>
</html>
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;
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 elementsvar y = cars.sort(); // The sort() method sorts arrays
The length Property
The length property of an array returns the length of an array (the number of array elements).Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The length property returns the length of an array.</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.length;
</script>
</body>
</html>
The length property is always one more than the highest array index.
Looping Array Elements
The best way to loop through an array, is using a "for" loop:Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The best way to loop through an array is using a standard for loop:</p>
<p id="demo"></p>
<script>
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>";
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Adding Array Elements
The easiest way to add a new element to an array is using the push method:Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The push method appends a new element to an array.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits;
function myFunction() {
fruits.push("Lemon");
document.getElementById("demo").innerHTML = fruits;
}
</script>
</body>
</html>
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The length property provides an easy way to append new elements to an array without using the push() method.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits;
function myFunction() {
fruits[fruits.length] = "Lemon";
document.getElementById("demo").innerHTML = fruits;
}
</script>
</body>
</html>
WARNING !
Adding elements with high indexes can create undefined "holes" in an array:
Adding elements with high indexes can create undefined "holes" in an array:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Adding elements with high indexes can create undefined "holes" in an array.</p>
<p id="demo"></p>
<script>
var fruits, text, fLen, i;
fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[6] = "Lemon";
fLen = fruits.length;
text = "";
for (i = 0; i < fLen; i++) {
text += fruits[i] + "<br>";
}
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p id="demo"></p>
<script>
var person = [];
person[0] = "John";
person[1] = "Doe";
person[2] = 46;
document.getElementById("demo").innerHTML =
person[0] + " " + person.length;
</script>
</body>
</html>"
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:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>If you use a named index, when accessing an array, JavaScript will redefined the array to a standard object, and some array methods and properties will produce undefined or incorrect results.</p>
<p id="demo"></p>
<script>
var person = [];
person["firstName"] = "John";
person["lastName"] = "Doe";
person["age"] = 46;
document.getElementById("demo").innerHTML =
person[0] + " " + person.length;
</script>
</body>
</html>
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(); // Badvar points = []; // Good
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Avoid using new Array(). Use [] instead.</p>
<p id="demo"></p>
<script>
//var points = new Array(40, 100, 1, 5, 25, 10);
var points = [40, 100, 1, 5, 25, 10];
document.getElementById("demo").innerHTML = points[0];
</script>
</body>
</html>
var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>Avoid using new Array().</p>
<p id="demo"></p>
<script>
var points = new Array(40);
document.getElementById("demo").innerHTML = points[0];
</script>
</body>
</html>
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":
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The typeof operator, when used on an array, returns object:</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = typeof fruits;
</script>
</body>
</html>
Solution 1:
To solve this problem ECMAScript 5 defines a new method Array.isArray():<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The new ECMASCRIPT 5 method isArray returns true when used on an array:</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = Array.isArray(fruits);
</script>
</body>
</html>
Solution 2:
To solve this problem you can create your own isArray() function: <!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>This "home made" isArray() function returns true when used on an array:</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = isArray(fruits);
function isArray(myArray) {
return myArray.constructor.toString().indexOf("Array") > -1;
}
</script>
</body>
</html>
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:<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>The instanceof operator returns true when used on an array:</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits instanceof Array;
</script>
</body>
</html>