JavaScript Functions
Example
function myFunction(p1, p2) {
return p1 * p2; //
The function returns the product of p1 and p2
}
»
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 the names listed in the function
definition.
Function arguments are the real 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:
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.
Example
Convert Fahrenheit to Celsius:
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:
Example
function toCelsius(fahrenheit) {
return (5/9) * (fahrenheit-32);
}
document.getElementById("demo").innerHTML
= toCelsius;
»
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:
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";
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