Node.js Tutorial - Node.js Functions

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JavaScript is a functional programming language, functions are fully typed objects that can be manipulated, extended, and passed around as data.

A normal function structure in JavaScript is defined as follows.

function functionName() {
   // function body
   // optional return; 
} 

All functions return a value in JavaScript.

In the absence of an explicit return statement, a function returns undefined.

function myData() { 
   return 123; 
} 
console.log(myData()); // 123 

function myValue() { 
} 
console.log(myValue()); // undefined 

Example

The following code shows how to create a function:

function hello(name) {
     console.log("hello " + name);
}
hello("CSS");

The code above generates the following result.

Note

To declare parameters for a function in JavaScript, list them in the parentheses.

There is no checking of these parameters at runtime:

function hello(name) {
     console.log("hello " + name);
}
hello();
hello("CSS", "HTML", "AAA", 4);

The code above generates the following result.

If too few parameters are passed into a function call, the resulting variables are assigned the value undefined.

If too many are passed in, the extras are simply unused.

All functions have a predefined array in the body called arguments.

It has all the values that were passed in to the function, and we can do extra checking on the parameter list.

Functions in JavaScript do not even need to have names:

var x = function (a, b) {
     return a + b;
}
console.log(x(10, 20));

The code above generates the following result.

The nameless functions are typically called anonymous functions.

Function Scope

Every time a function is called, a new variable scope is created.

Variables declared in the parent scope are available to that function.

Variables declared within the new scope are not available when the function exits.

Consider the following code:

var pet = 'cat';

function myMethod() {
     var pet = 'dog';
     console.log(pet);
 }

myMethod();
console.log(pet);

The code above generates the following result.

Combining this scoping with anonymous functions is better way to use private variables that will disappear when the anonymous function exits.

Example 2

Here's a contrived example to compute the volume of a cone:

var height = 5;
var radius = 3;
var volume;
// declare and immediately call anonymous function to create scope
(function () {/*from w  w  w  . j  av a  2 s  . c  o m*/
     var pir2 = Math.PI * radius * radius;   // temp var
     volume = (pir2 * height) / 3;
})();

console.log(volume);

The code above generates the following result.

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