How do you create a function in JavaScript?

  Functions in JavaScript

JavaScript function is code that perform a particular task.

A JavaScript function is executed when its called. 

Eg. 

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: {}

functionname(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.

 Study pdf : JavaScript Notes PDF

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.

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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

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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);

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The () Operator Invokes the Function

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

Example

Accessing a function without () will return the function definition:

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

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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:

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

 

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JavaScript Video Tutorial 

(Fast Introduction) 

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