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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Functions</h2>
<p>This example calls a function which performs a calculation, and returns the result:</p>
<p id="demo"></p>
<script>
function myFunction(p1, p2) {
return p1 * p2;
}
document.getElementById("demo").innerHTML = myFunction(4, 3);
</script>
</body>
</html>
Result:
JavaScript Functions
This example calls a function which performs a calculation, and returns the result:
12
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.
A Function is much the same as a Procedure or a Subroutine, in other programming languages.
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:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Functions</h2>
<p>This example calls a function which performs a calculation and returns the result:</p>
<p id="demo"></p>
<script>
var x = myFunction(4, 3);
document.getElementById("demo").innerHTML = x;
function myFunction(a, b) {
return a * b;
}
</script>
</body>
</html>
Result:
JavaScript Functions
This example calls a function which performs a calculation and returns the result:
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.
Example
Convert Fahrenheit to Celsius:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Functions</h2>
<p>This example calls a function to convert from Fahrenheit to Celsius:</p>
<p id="demo"></p>
<script>
function toCelsius(f) {
return (5/9) * (f-32);
}
document.getElementById("demo").innerHTML = toCelsius(77);
</script>
</body>
</html>
Result:
JavaScript Functions
This example calls a function to convert from Fahrenheit to Celsius:
25
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 object instead of the function result.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Functions</h2>
<p>Accessing a function without () will return the function definition instead of the function result:</p>
<p id="demo"></p>
<script>
function toCelsius(f) {
return (5/9) * (f-32);
}
document.getElementById("demo").innerHTML = toCelsius;
</script>
</body>
</html>
Result:
JavaScript Functions
Accessing a function without () will return the function definition instead of the function result:
function toCelsius(f) { return (5/9) * (f-32); }
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:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Functions</h2>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
"The temperature is " + toCelsius(77) + " Celsius";
function toCelsius(fahrenheit) {
return (5/9) * (fahrenheit-32);
}
</script>
</body>
</html>
Result:
JavaScript Functions
The temperature is 25 Celsius
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Functions</h2>
<p>Outside myFunction() carName is undefined.</p>
<p id="demo1"></p>
<p id="demo2"></p>
<script>
myFunction();
function myFunction() {
let carName = "Volvo";
document.getElementById("demo1").innerHTML =
typeof carName + " " + carName;
}
document.getElementById("demo2").innerHTML =
typeof carName;
</script>
</body>
</html>
Result:
JavaScript Functions
Outside myFunction() carName is undefined.
string Volvo
undefined
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.