How to use javascript in website

Use JavaScript within a webpage

Take your webpages to the next level by harnessing JavaScript. Learn in this article how to trigger JavaScript right from your HTML documents.

Prerequisites:	You need to be familiar with how to create a basic HTML document.
Objective:	Learn how to trigger JavaScript in your HTML file, and learn the most important best practices for keeping JavaScript accessible.

About JavaScript

JavaScript is a programming language mostly used client-side to make webpages interactive. You can create amazing webpages without JavaScript, but JavaScript opens up a whole new level of possibilities.

Note: In this article we’re going over the HTML code you need to make JavaScript take effect. If you want to learn JavaScript itself, you can start with our JavaScript basics article. If you already know something about JavaScript or if you have a background with other programming languages, we suggest you jump directly into our JavaScript Guide.

How to trigger JavaScript from HTML

Within a browser, JavaScript doesn’t do anything by itself. You run JavaScript from inside your HTML webpages. To call JavaScript code from within HTML, you need the element. There are two ways to use script , depending on whether you’re linking to an external script or embedding a script right in your webpage.

Linking an external script

Usually, you’ll be writing scripts in their own .js files. If you want to execute a .js script from your webpage, just use with an src attribute pointing to the script file, using its URL:

script src="path/to/my/script.js"> script> 

Writing JavaScript within HTML

You may also add JavaScript code between tags rather than providing an src attribute.

script> window.addEventListener("load", () =>  console.log("This function is executed once the page is fully loaded"); >); script> 

That’s convenient when you just need a small bit of JavaScript, but if you keep JavaScript in separate files you’ll find it easier to

• focus on your work
• write self-sufficient HTML
• write structured JavaScript applications

Use scripting accessibly

Accessibility is a major issue in any software development. JavaScript can make your website more accessible if you use it wisely, or it can become a disaster if you use scripting without care. To make JavaScript work in your favor, it’s worth knowing about certain best practices for adding JavaScript:

• Make all content available as (structured) text. Rely on HTML for your content as much as possible. For example, if you’ve implemented a nice JavaScript progress bar, make sure to supplement it with matching text percentages inside the HTML. Likewise, your drop-down menus should be structured as unordered lists of links.
• Make all functionality accessible from the keyboard.
  • Let users Tab through all controls (e.g., links and form input) in a logical order.
  • If you use pointer events (like mouse events or touch events), duplicate the functionality with keyboard events.
  • Test your site using a keyboard only.
  • At a minimum, leave a short message with like this:
  • Ideally, replicate the JavaScript functionality with HTML and server-side scripting when possible.
  • If you’re only looking for simple visual effects, CSS can often get the job done even more intuitively.
  • Since almost everybody does have JavaScript enabled,

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  How To Add JavaScript to HTML

  JavaScript, also abbreviated to JS, is a programming language used in web development. As one of the core technologies of the web alongside HTML and CSS, JavaScript is used to make webpages interactive and to build web apps. Modern web browsers, which adhere to common display standards, support JavaScript through built-in engines without the need for additional plugins.

  When working with files for the web, JavaScript needs to be loaded and run alongside HTML markup. This can be done either inline within an HTML document or in a separate file that the browser will download alongside the HTML document.

  This tutorial will go over how to incorporate JavaScript into your web files, both inline into an HTML document and as a separate file.

  Adding JavaScript into an HTML Document

  You can add JavaScript code in an HTML document by employing the dedicated HTML tag that wraps around JavaScript code.

  The tag can be placed in the section of your HTML or in the section, depending on when you want the JavaScript to load.

  Generally, JavaScript code can go inside of the document section in order to keep them contained and out of the main content of your HTML document.

  However, if your script needs to run at a certain point within a page’s layout — like when using document.write to generate content — you should put it at the point where it should be called, usually within the section.

  Let’s consider the following blank HTML document with a browser title of Today’s Date :

  DOCTYPE html> html lang="en-US"> head> meta charset="UTF-8"> meta name="viewport" content="width=device-width, initial-scale=1"> title>Today's Datetitle> head> body> body> html> 

  Right now, this file only contains HTML markup. Let’s say we would like to add the following JavaScript code to the document:

  let d = new Date(); alert("Today's date is " + d); 

  This will enable the webpage to display an alert with the current date regardless of when the user loads the site.

  In order to achieve this, we will add a tag along with some JavaScript code into the HTML file.

  To begin with, we’ll add the JavaScript code between the tags, signalling the browser to run the JavaScript script before loading in the rest of the page. We can add the JavaScript below the tags, for instance, as shown below:

  DOCTYPE html> html lang="en-US"> head> meta charset="UTF-8"> meta name="viewport" content="width=device-width, initial-scale=1"> title>Today's Datetitle> script> let d = new Date(); alert("Today's date is " + d); script> head> body> body> html> 

  Once you load the page, you will receive an alert similar to this:

  

  If we were modifying what is shown in the body of the HTML, we would need to implement that after the section so that it displays on the page, as in the example below:

  DOCTYPE html> html lang="en-US"> head> meta charset="UTF-8"> meta name="viewport" content="width=device-width, initial-scale=1"> title>Today's Datetitle> head> body> script> let d = new Date(); document.body.innerHTML = "Today's date is " + d + ">" script> body> html> 

  The output for the above HTML document loaded through a web browser would look similar to the following:

  

  Scripts that are small or that run only on one page can work fine within an HTML file, but for larger scripts or scripts that will be used on many pages, it is not a very effective solution because including it can become unwieldy or difficult to read and understand. In the next section, we’ll go over how to handle a separate JavaScript file in your HTML document.

  Working with a Separate JavaScript File

  In order to accommodate larger scripts or scripts that will be used across several pages, JavaScript code generally lives in one or more js files that are referenced within HTML documents, similarly to how external assets like CSS are referenced.

  The benefits of using a separate JavaScript file include:

  • Separating the HTML markup and JavaScript code to make both more straightforward
  • Separate files makes maintenance easier
  • When JavaScript files are cached, pages load more quickly

  To demonstrate how to connect a JavaScript document to an HTML document, let’s create a small web project. It will consist of script.js in the js/ directory, style.css in the css/ directory, and a main index.html in the root of the project.

  project/ ├── css/ | └── style.css ├── js/ | └── script.js └── index.html 

  We can start with our previous HTML template from the section above:

  DOCTYPE html> html lang="en-US"> head> meta charset="UTF-8"> meta name="viewport" content="width=device-width, initial-scale=1"> title>Today's Datetitle> head> body> body> html> 

  Now, let’s move our JavaScript code that will show the date as an header to the script.js file:

  let d = new Date(); document.body.innerHTML = "
  Today's date is " + d + "
  " 

  We can add a reference to this script to the section, with the following line of code:

  script src="js/script.js">/script> 

  The tag is pointing to the script.js file in the js/ directory of our web project.

  Let’s consider this line in the context of our HTML file, in this case, within the section:

  DOCTYPE html> html lang="en-US"> head> meta charset="UTF-8"> meta name="viewport" content="width=device-width, initial-scale=1"> title>Today's Datetitle> head> body> script src="js/script.js">script> body> html> 

  Finally, let’s also edit the style.css file by adding a background color and style to the header:

  body  background-color: #0080ff; > h1  color: #fff; font-family: Arial, Helvetica, sans-serif; > 

  We can reference that CSS file within the section of our HTML document:

  DOCTYPE html> html lang="en-US"> head> meta charset="UTF-8"> meta name="viewport" content="width=device-width, initial-scale=1"> title>Today's Datetitle> link rel="stylesheet" href="css/style.css"> head> body> script src="js/script.js">script> body> html> 

  Now, with the JavaScript and CSS in place we can load the index.html page into the web browser of our choice. We should see a page that looks similar to the following:

  

  Now that we’ve placed the JavaScript in a file, we can call it in the same way from additional web pages and update them all in a single location

  Conclusion

  This tutorial went over how to incorporate JavaScript into your web files, both inline into an HTML document and as a separate .js file.

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  Tutorial Series: How To Code in JavaScript

  JavaScript is a high-level, object-based, dynamic scripting language popular as a tool for making webpages interactive.

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