Posted By
Bajarangi soft ,
Posted On 21-09-2020
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 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: {}
Syntax
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)
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(1)
<p>This example calls a function which performs a calculation and returns the result:</p>
<p id="demo"></p>
<script>
var x = Multiplication(4, 3);
document.getElementById("demo").innerHTML = x;
function Multiplication(a, b) {
return a * b;
}
</script>
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.
<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>
Local Variables
Variables declared within a JavaScript function, become LOCAL to the function.
Local variables can only be accessed from within the function.
<p>Outside myFunction() bikeName is undefined.</p>
<p id="demo1"></p>
<p id="demo2"></p>
<script>
myFunction();
function myFunction() {
var bikeName = "Bike";
document.getElementById("demo1").innerHTML =
typeof bikeName + " " + bikeName;
}
document.getElementById("demo2").innerHTML =
typeof bikeName;
</script>
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.
Complete code to define function in Java Script
<!DOCTYPE html>
<html>
<head>
<title>JavaScript Functions</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css">
</head>
<style>
h3{
color:red;
}
</style>
<body>
<div class="container">
<br>
<br>
<div class="text-center">
<h1>JavaScript Functions</h1>
</div>
<br>
<div class="well">
<h1>Multiplication Functions</h1>
<h3 id="demo"></h3>
</div>
<br>
</div>
</body>
</html>
<script>
var c;
var x = Multiplication(5, 3);
document.getElementById("demo").innerHTML = x;
function Multiplication(a, b) { //This example calls a function which performs a calculation and returns the result
return a * b;
}
</script>