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).

This example calls a function which performs a calculation, and returns the result:

               <!-- Just an empty paragraph, where the result of the calculation of the following function will appear -->
                <p id="demo"></p>

                    // This is a function that calculates the multiplication of two variables
                    function myFunction(p1, p2) {
                        return p1 * p2;

                    // After calculation we invoke the function and place its result into a paragraph with id = "demo" 
                    document.getElementById("demo").innerHTML = myFunction(4, 3);

Live preview

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 name(parameter1, parameter2, parameter3) {
                        // code to be executed

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":

                      var x = myFunction(4, 3);   // Function is called, return value will end up in x
                      function myFunction(a, b) {
                        return a * b;             // Function returns the product of a and b
                      // The result in x will be 12

Why Functions?

You can reuse code: Define the code once, and use it many times.

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

                      function toCelsius(fahrenheit) {
                        return (5/9) * (fahrenheit-32);
                      document.getElementById("demo").innerHTML = toCelsius(77);

The () Operator Invokes the Function

Using the example above, toCelsius refers to the function object, and toCelsius() refers to the function result.

Accessing a function without () will return the function definition instead of the function result:

                      function toCelsius(fahrenheit) {
                        return (5/9) * (fahrenheit-32);
                      document.getElementById("example-2").innerHTML = toCelsius;

Live preview

If you do not call the function with () here is the result you get from the example above - instead of the calculated value, the function definition itself will be inserted in the "example-2" paragraph.

Functions Used as Variable Values

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


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

                    var x = toCelsius(77);
                    var text = "The temperature is " + x + " Celsius";

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

                      var text = "The temperature is " + toCelsius(77) + " Celsius";

Note: 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.

                      function myFunction() {
                        var carName = "Volvo";
                      // Result "string Volvo", because we can access the variable carName inside the function
                        document.getElementById("demo1").innerHTML = typeof carName + " " + carName;
                      // Result undefined, because we can not access the variable carName outside the function
                      document.getElementById("demo2").innerHTML = typeof 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.

Exercises - test your knowledge

Exercise 1

Execute the function named myFunction.

                    function myFunction() {
                      alert("Hello World!");

                        function myFunction() {
                          alert("Hello World!");

Exercise 2

Create a function called "myFunction" which alert "Hello World!".

                      function myFunction() {
                        alert("Hello World!");

Exercise 3

Make the function return "Hello".

                      function myFunction() {

                        function myFunction() {
                          return "Hello";

Exercise 4

Make the function display "Hello" in the inner HTML of an element with the ID "exercise".

                      function myFunction() {

                        function myFunction() {
                          document.getElementById("exercise").innerHTML = "Hello";

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Michal Szymanski
Co-Founder at MDBootstrap & BrandFlow. Entrepreneur, web developer, UI/UX designer, marketing analyst. Dancer and nerd in one.