In this tutorial we will use a script to display arrays inside a <p> element with id="demo":
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.
Create an array, and assign values to it:
Spaces and line breaks are not important. A declaration can span multiple lines:
|Never put a comma after the last element (like "BMW",). The effect is inconsistent across browsers.|
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:
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.
The following example also creates an Array, and assigns values to it:
|The two examples above do exactly the same. There is no need to use new
For simplicity, readability and execution speed, use the first one (the array literal method).
You refer to an array element by referring to the index number.
This statement accesses the value of the first element in cars:
This statement modifies the first element in cars:
| is the first element in an array.  is the second. Array indexes start with 0.|
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:
Arrays use numbers to access its "elements". In this example, person returns John:
Objects use names to access its "members". In this example, person.firstName returns John:
Array methods are covered in the next chapter.
The length property of an array returns the length of an array (the number of array elements).
|The length property is always one more than the highest array index.|
The easiest way to add a new element to an array is using the push method:
New element can also be added to an array using the length property:
Adding elements with high indexes can create undefined "holes" in an array:
The best way to loop through an array, is using a "for" loop:
Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative arrays (or hashes).
After that, all array methods and properties will produce incorrect results.
|Arrays are a special kind of objects, with numbered indexes.|
Use  instead.
These two different statements both create a new empty array named points:
These two different statements both create a new array containing 6 numbers:
The new keyword complicates your code and produces nasty side effects:
What if I remove one of the elements?
A common question is: How do I know if a variable is an array?
To solve this problem you can create your own isArray() function:
The function above always returns true if the argument is an array.
Or more precisely: it returns true if the object prototype of the argument is "[object array]".