JavaScript Functions and JavaScript Function Syntax and Function Invocation and Function Return and example code

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 in JavaScript is a reusable block of code that performs a specific task. You define it once, and then you can run (or “call”) it whenever you need that task done in your program.

 // Function to compute the product of p1 and p2

function myFunction(p1, p2) {
  return p1 * 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 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)

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"

// Function is called, the return value will end up in x
let x = myFunction(4, 3);

function myFunction(a, b) {
// Function returns the product of a and b
  return a * b;
}

Why Functions?

• With functions you can reuse code
• You can write code that can be used many times.
• You can use the same code with different arguments, to produce different results.
• Functions can be used multiple times, reducing redundancy.
• Break down complex problems into manageable pieces.
• Manage complexity by hiding implementation details.
• Can call themselves to solve problems recursively.

Function Definition

Before, using a user-defined function in JavaScript we have to create one. We can use the above syntax to create a function in JavaScript.

A function definition is sometimes also termed a function declaration or function statement. Below are the rules for creating a function in JavaScript:

• Every function should begin with the keyword function followed by,
• A user-defined function name that should be unique,
• A list of parameters enclosed within parentheses and separated by commas,
• A list of statements composing the body of the function enclosed within curly braces {}.