How to call a JavaScript function in HTML?

JavaScript Functions

A JavaScript function is a block of code designed to perform a particular task.

A JavaScript function is executed when «something» invokes it (calls it).

Example

JavaScript Function Syntax

A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().

Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).

The parentheses may include parameter names separated by commas:
(parameter1, parameter2, . )

The code to be executed, by the function, is placed inside curly brackets: <>

Function parameters are listed inside the parentheses () in the function definition.

Function arguments are the values received by the function when it is invoked.

Inside the function, the arguments (the parameters) behave as local variables.

Function Invocation

The code inside the function will execute when «something» invokes (calls) the function:

• When an event occurs (when a user clicks a button)
• When it is invoked (called) from JavaScript code
• Automatically (self invoked)

You will learn a lot more about function invocation later in this tutorial.

Function Return

When JavaScript reaches a return statement, the function will stop executing.

If the function was invoked from a statement, JavaScript will «return» to execute the code after the invoking statement.

Functions often compute a return value. The return value is «returned» back to the «caller»:

Example

Calculate the product of two numbers, and return the result:

// Function is called, the return value will end up in x
let x = myFunction(4, 3);

function myFunction(a, b) // Function returns the product of a and b
return a * b;
>

Why Functions?

With functions you can reuse code

You can write code that can be used many times.

You can use the same code with different arguments, to produce different results.

The () Operator

The () operator invokes (calls) the function:

Example

Convert Fahrenheit to Celsius:

function toCelsius(fahrenheit) <
return (5/9) * (fahrenheit-32);
>

Accessing a function with incorrect parameters can return an incorrect answer:

Example

function toCelsius(fahrenheit) <
return (5/9) * (fahrenheit-32);
>

Accessing a function without () returns the function and not the function result:

Example

function toCelsius(fahrenheit) <
return (5/9) * (fahrenheit-32);
>

Note

As you see from the examples above, toCelsius refers to the function object, and toCelsius() refers to the function result.

Functions Used as Variable Values

Functions can be used the same way as you use variables, in all types of formulas, assignments, and calculations.

Example

Instead of using a variable to store the return value of a function:

You can use the function directly, as a variable value:

You will learn a lot more about functions later in this tutorial.

Local Variables

Variables declared within a JavaScript function, become LOCAL to the function.

Local variables can only be accessed from within the function.

Example

// code here can NOT use carName

function myFunction() let carName = «Volvo»;
// code here CAN use carName
>

// code here can NOT use carName

Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.

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How to call a JavaScript function in HTML?

It is not difficult to call a JavaScript function in HTML using the different ways or methods in JavaScript. Using HTML and JavaScript, web developers can structure a web page with various features and attributes. Also, HTML and CSS can structure a page and provide a web page with an attractive appearance and responsive look. We will get to learn how to use HTML with JavaScript to call the JS function from this article.

What is the need for JavaScript to call a function in HTML?

JavaScript allows developers to give a web page an interactive look for users and guides them to the correct information, while users can use the HTML ad CSS to style and add a good structure to their web page.

Also, users can use JavaScript functions to create websites using HTML that responds to everyday user events like key presses, clicks, page reloads, and mouse hovering. That is why almost every web developer prefers to use HTML and JavaScript and call the JavaScript function in HTML.

There are different ways to create an HTML file and call the JavaScript function in HTML. This article will discuss the below:

Method 1: Using Script Tag

Code Snippet:

Method 2: Using HTML event attributes like onClick(), onBlur(), onSubmit():

In this method, users first create a button using the input tag. After they make an HTML button, the browser will display the button on the screen, and by clicking the button, users can see a dialog box that pops up on the user’s screen that the developer has already declared in the JavaScript function func() with an alert() function.

The clickEvent() function lets them execute the alert() when the button gets clicked by using the onclick() method.

Some on usefull Event Attributes

• onblur
• onchange
• onsubmit
• onkeydown
• onclick
• ondblclick
• onmouseover

for a complete list of HTML Event Attributes click here

Code Snippet:

   function func()  
Click the button below to get a notification from the website and see the action
 

Explanation:

In the program mentioned above, we have built a simple HTML document. Within the head section of the HTML document, inside the script tags « » we have used a user-defined function called the func(). We can build a link or a button and use an onclick() event associated with them to call the «func» function.

We have used the onclick attribute and displayed a button for calling the function. When a user clicks on the given button, the browser executes the function and displays the alert message, as displayed in the output console.

Method 3: Using href attribut of anchor tag

Code Snippet:

   function myfunc() 
Click here to call JavaScript function
  

Method 4: Using external files to call a function in HTML:

Here, we use the second method of calling a JavaScript function in HTML. This method involves an external file (.js file), and we call the JS function in HTML through that external file.

To create an external JavaScript file, we need to define our function inside it and save the file with the «.js» extension. Let us understand how it works with an example given below:

Code Snippet:

      
Click here (button) below to call the function
 

External JavaScript File

Explanation:

In the above code snippet, we first created an external JavaScript file. Inside the file, we defined the function and saved it with a file extension «.js.» After creating the JavaScript file, we built a simple HTML document and linked the previous JavaScript file using .

Since we have saved the HTML document and the JavaScript file inside the same folder, we have to name the JavaScript file in the «src» attribute rather than adding the full path in the head section.

Then we used the tag to display some text using a button. Lastly, we used the onclick attribute with the button to call the JavaScript function, and when a user clicks on the button, the browser invokes the JS function and displays the alert message, as shown in the output console.

We hope this article has catered to an idea of how to call a JavaScript function in HTML. Also, this article added to some advantages and the need to use HTML with JavaScript for developing web pages. The article briefly explains both these methods mentioned above with the respective code snippets of how we can invoke a function in HTML.

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Calling functions in javascript html

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Calling functions in javascript html

Learn Latest Tutorials

Preparation

Trending Technologies

B.Tech / MCA

Javatpoint Services

JavaTpoint offers too many high quality services. Mail us on h[email protected], to get more information about given services.

• Website Designing
• Website Development
• Java Development
• PHP Development
• WordPress
• Graphic Designing
• Logo
• Digital Marketing
• On Page and Off Page SEO
• PPC
• Content Development
• Corporate Training
• Classroom and Online Training
• Data Entry

Training For College Campus

JavaTpoint offers college campus training on Core Java, Advance Java, .Net, Android, Hadoop, PHP, Web Technology and Python. Please mail your requirement at [email protected].
Duration: 1 week to 2 week

Like/Subscribe us for latest updates or newsletter

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