JavaScript Functions
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).
A function is a group of reusable code which can be called anywhere in your program. This eliminates the need of writing the same code again and again. It helps programmers in writing modular codes. Functions allow a programmer to divide a big program into a number of small and manageable functions.
Like any other advanced programming language, JavaScript also supports all the features necessary to write modular code using functions. You must have seen functions like alert() and write() in the earlier chapters. We were using these functions again and again, but they had been written in core JavaScript only once.
Function Definition
Before we use a function, we need to define it. The most common way to define a function in JavaScript is by using the function keyword, followed by a unique function name, a list of parameters (that might be empty), and a statement block surrounded by curly braces.
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. A Function is much the same as a Procedure or a Subroutine, in other programming languages.
function myFunction(p1, p2) {
return p1 * p2; // The function returns the product of p1 and p2
}
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)
function myFunction(p1, p2) {
return p1 * p2; // The function returns the product of p1 and p2
}
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
}
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);
Incrementing
The increment operator (++) increments numbers.
var x = 5;
x++;
var z = x;
The () Operator Invokes the Function
Accessing a function without () will return the function object instead of the function result.
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.
You can use the function directly, as a variable value:
It's a good programming practice to declare all variables at the beginning of a script.
var x = toCelsius(77);
var text = "The temperature is " + x + " Celsius";
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var text = "The temperature is " + toCelsius(77) + " Celsius";
Local Variables
Variables declared within a JavaScript function, become LOCAL to the function. Local variables can only be accessed from within the function.
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
