Javascript in local html file

JavaScript Where To

In HTML, JavaScript code is inserted between tags.

Example

Old JavaScript examples may use a type attribute: .
The type attribute is not required. JavaScript is the default scripting language in HTML.

JavaScript Functions and Events

A JavaScript function is a block of JavaScript code, that can be executed when «called» for.

For example, a function can be called when an event occurs, like when the user clicks a button.

You will learn much more about functions and events in later chapters.

JavaScript in or

You can place any number of scripts in an HTML document.

Scripts can be placed in the , or in the section of an HTML page, or in both.

JavaScript in

In this example, a JavaScript function is placed in the section of an HTML page.

The function is invoked (called) when a button is clicked:

Example

Demo JavaScript in Head

JavaScript in

In this example, a JavaScript function is placed in the section of an HTML page.

The function is invoked (called) when a button is clicked:

Example

Demo JavaScript in Body

Placing scripts at the bottom of the element improves the display speed, because script interpretation slows down the display.

External JavaScript

Scripts can also be placed in external files:

External file: myScript.js

External scripts are practical when the same code is used in many different web pages.

JavaScript files have the file extension .js.

To use an external script, put the name of the script file in the src (source) attribute of a tag:

Example

You can place an external script reference in or as you like.

The script will behave as if it was located exactly where the tag is located.

External scripts cannot contain tags.

External JavaScript Advantages

Placing scripts in external files has some advantages:

• It separates HTML and code
• It makes HTML and JavaScript easier to read and maintain
• Cached JavaScript files can speed up page loads

To add several script files to one page — use several script tags:

Example

External References

An external script can be referenced in 3 different ways:

• With a full URL (a full web address)
• With a file path (like /js/)
• Without any path

This example uses a full URL to link to myScript.js:

Example

This example uses a file path to link to myScript.js:

Example

This example uses no path to link to myScript.js:

Example

You can read more about file paths in the chapter HTML File Paths.

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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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