Now that you know about the basic primitive data types in JavaScript, it's time to move to a more powerful data structure—the array.
So, what is an array? It's simply a list (a sequence) of values. Instead of using one variable to store one value, you can use one array variable to store any number of values as elements of the array.
To declare a variable that contains an empty array, you use square brackets with nothing between them:
> var a = [];
To define an array that has three elements, you do this:
> var a = [1, 2, 3];
When you simply type the name of the array in the console, you get the contents of your array:
> a;
[1, 2, 3]
Now the question is how to access the values stored in these array elements. The elements contained in an array are indexed with consecutive numbers starting from zero. The first element has index (or position) 0, the second has index 1, and so on. Here's the three-element array from the previous example:
Index |
Value |
---|---|
|
|
|
|
|
|
To access an array element, you specify the index of that element inside square brackets. So, a[0]
gives you the first element of the array a
, a[1]
gives you the second, and so on:
> a[0]; 1 > a[1]; 2
Using the index, you can also update the values of the elements of the array. The next example updates the third element (index 2) and prints the contents of the new array:
> a[2] = 'three'; "three" > a; [1, 2, "three"]
You can add more elements by addressing an index that didn't exist before:
> a[3] = 'four'; "four" > a; [1, 2, "three", "four"]
If you add a new element, but leave a gap in the array, those elements in between don't exist and return the undefined
value if accessed. Check out this example:
> var a = [1, 2, 3]; > a[6] = 'new'; "new" > a; [1, 2, 3, undefined x 3, "new"]
To delete an element, you use the delete
operator. However, after the deletion, the length of the array does not change. In a sense, you get a hole in the array:
> var a = [1, 2, 3]; > delete a[1]; true > a; [1, undefined, 3] > typeof a[1]; "undefined"
Arrays can contain all types of values, including other arrays:
> var a = [1, "two", false, null, undefined]; > a; [1, "two", false, null, undefined] > a[5] = [1, 2, 3]; [1, 2, 3] > a; [1, "two", false, null, undefined, Array[3]]
The Array[3] in the result is clickable in the console and it expands the array values. Let's see an example where you have an array of two elements, both of them being other arrays:
> var a = [[1, 2, 3], [4, 5, 6]];
> a;
[Array[3], Array[3]]
The first element of the array is a[0]
, and it's also an array:
> a[0];
[1, 2, 3]
To access an element in the nested array, you refer to the element index in another set of square brackets:
> a[0][0]; 1 > a[1][2]; 6
Note that you can use the array notation to access individual characters inside a string:
> var s = 'one'; > s[0]; "o" > s[1]; "n" > s[2]; "e"
There are more ways to have fun with arrays (and you'll get to those in Chapter 4, Objects), but let's stop here for now, remembering that:
3.145.17.20