JAVASCRIPT

  JavaScript is one of the 3 languages all web developers must learn:

   1. HTML to define the content of web pages

   2. CSS to specify the layout of web pages

   3. JavaScript to program the behavior of web pages

JavaScript is a lightweight, interpreted programming language. It is designed for creating network-centric applications. It is complimentary to and integrated with Java. JavaScript is very easy to implement because it is integrated with HTML. It is open and cross-platform.

What is JavaScript doing on your page?

Here we'll start actually looking at some code, and while doing so explore what actually happens when you run some JavaScript in your page.

Let's briefly recap the story of what happens when you load a web page in a browser (first talked about in our How CSS works article). When you load a web page in your browser, you are running your code (the HTML, CSS, and JavaScript) inside an execution environment (the browser tab). This is like a factory that takes in raw materials (the code) and outputs a product (the web page).

The JavaScript is executed by the browser's JavaScript engine, after the HTML and CSS have been assembled and put together into a web page. This ensures that the structure and style of the page are already in place by the time the JavaScript starts to run.

This is a good thing, as a very common use of JavaScript is to dynamically modify HTML and CSS to update a user interface, via the Document Object Model API (as mentioned above). If the JavaScript loaded and tried to run before the HTML and CSS was there to affect, then errors would occur.

Browser security

Each browser tab is its own separate bucket for running code in (these buckets are called "execution environments" in technical terms) — this means that in most cases the code in each tab is run completely separately, and the code in one tab cannot directly affect the code in another tab — or on another website. This is a good security measure — if this were not the case, then pirates could start writing code to steal information from other websites, and other such bad things.

Note: There are ways to send code and data between different websites/tabs in a safe manner, but these are advanced techniques that we won't cover in this course.

JavaScript running order

When the browser encounters a block of JavaScript, it generally runs it in order, from top to bottom. This means that you need to be careful what order you put things in. For example, let's return to the block of JavaScript we saw in our first example:

Here we are selecting a text paragraph (line 1), then attaching an event listener to it (line 3) so that when the paragraph is clicked, the updateName() code block (lines 5–8) is run. The updateName() code block (these types of reusable code block are called "functions") asks the user for a new name, and then inserts that name into the paragraph to update the display.

If you swapped the order of the first two lines of code, it would no longer work — instead, you'd get an error returned in the browser developer console — TypeError: para is undefined. This means that the para object does not exist yet, so we can't add an event listener to it.

Note: This is a very common error — you need to be careful that the objects referenced in your code exist before you try to do stuff to them.

Interpreted versus compiled code

You might hear the terms interpreted and compiled in the context of programming. JavaScript is an interpreted language — the code is run from top to bottom and the result of running the code is immediately returned. You don't have to transform the code into a different form before the browser runs it.

Compiled languages on the other hand are transformed (compiled) into another form before they are run by the computer. For example C/C++ are compiled into assembly language that is then run by the computer.

Both approaches have different advantages, which we won't discuss at this point.

Server-side versus client-side code

You might also hear the terms server-side and client-side code, specially in the context of web development. Client-side code is code that is run on the user's computer — when a web page is viewed, the page's client-side code is downloaded, then run and displayed by the browser. In this JavaScript module we are explicitly talking about client-side JavaScript.

Server-side code on the other hand is run on the server, then its results are downloaded and displayed in the browser. Examples of popular server-side web languages include PHP, Python, Ruby, and ASP.NET. And JavaScript! JavaScript can also be used as a server-side language, for example in the popular Node.js environment — you can find more out about server-side JavaScript in our Dynamic Websites – Server-side programming topic.

The word dynamic is used to describe both client-side JavaScript, and server-side languages — it refers to the ability to update the display of a web page/app to show different things in different circumstances, generating new content as required. Server-side code dynamically generates new content on the server, e.g. pulling data from a database, whereas client-side JavaScript dynamically generates new content inside the browser on the client, e.g. creating a new HTML table, inserting data requested from the server into it, then displaying the table in a web page shown to the user. The meaning is slightly different in the two contexts, but related, and both approaches (server-side and client-side) usually work together.

A web page with no dynamically updating content is referred to as static — it just shows the same content all the time.

 

 

 

How do you add JavaScript to your page?

JavaScript is applied to your HTML page in a similar manner to CSS. Whereas CSS uses <link> elements to apply external stylesheets and <style> elements to apply internal stylesheets to HTML, JavaScript only needs one friend in the world of HTML — the <script> element. Let's learn how this works.

Internal JavaScript

1. First of all, make a local copy of our example file apply-javascript.html. Save it in a directory somewhere sensible.
2. Open the file in your web browser and in your text editor. You'll see that the HTML creates a simple web page containing a clickable button.
3. Next, go to your text editor and add the following just before your closing </body> tag:

          

Now we'll add some JavaScript inside our <script> element to make the page do something more interesting — add the following code just below the "// JavaScript goes here" line:

Save your file and refresh the browser — now you should see that when you click the button, a new paragraph is generated and placed below.

External JavaScript

This works great, but what if we wanted to put our JavaScript in an external file? Let's explore this now.

1. First, create a new file in the same directory as your sample HTML file. Call it script.js — make sure it has that .js filename extension, as that's how it is recognized as JavaScript.
2. Next, copy all of the script out of your current <script> element and paste it into the .js file. Save that file.
3. Now replace your current <script> element with the following:
4. <script src="script.js"</script>
5. Save and refresh your browser, and you should see the same thing! It works just the same, but now we've got the JavaScript in an external file. This is generally a good thing in terms of organizing your code, and making it reusable across multiple HTML files. Plus the HTML is easier to read without huge chunks of script dumped in it.

Note: You can see this version on GitHub as apply-javascript-external.html and script.js (see it live too).

Inline JavaScript handlers

Note that sometimes you'll come across bits of actual JavaScript code living inside HTML. It might look something like this:

 

Using a pure JavaScript construct allows you to select all the buttons using one instruction. The code we used above to serve this purpose looks like this:

This might look a bit longer than the onclick attribute, but this will work for all buttons no matter how many are on the page, and how many are added or removed. The JavaScript does not need to be changed.

Note: Try editing your version of apply-javascript.html and add a few more buttons into the file. When you reload, you should find that all of the buttons when clicked will create a paragraph. Neat, huh?

As with HTML and CSS, it is possible to write comments into your JavaScript code that will be ignored by the browser, and exist simply to provide instructions to your fellow developers on how the code works (and you, if you come back to your code after 6 months and can't remember what you did). Comments are very useful, and you should use them often, particularly for larger applications. There are two types

• A single line comment is written after a double forward slash (//), e.g.

• A multi-line comment is written between the strings /* and */, e.g.

So for example, we could annotate our last demo's JavaScript with comments like so:

 

    More Learn to: