Node.js Tutorial - Node.js Higher Order Function

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Immediately Executing Function

We can execute a function immediately after you define it.

Simply wrap the function in parentheses () and invoke it.

(function myData() {
   console.log('myData was executed!'); 
})(); 

An immediately executing function creates a new variable scope.

An if, else, or while does not create a new variable scope in JavaScript.

var myData = 123; 
if (true) {
   var myData = 456; 
} 
console.log(myData); // 456; 

The only recommended way of creating a new variable scope in JavaScript is using a function.

The following code shows how to create a new variable scope with an immediately executing function.

var myData = 123; 
if (true) { 
    (function () { // create a new scope 
        var myData = 456;
     })(); 
} 
console.log(myData); // 123; 

Anonymous Function

A function without a name is called an anonymous function.

In JavaScript, you can assign a function to a variable.

If you are going to use a function as a variable, you don't need to name the function.

The following code shows two ways of defining a function inline.

Both of these methods are equivalent.

var foo1 = function namedFunction() {
   console.log('foo1'); 
} /*w  w w .  j  ava 2s.  c o  m*/
foo1(); // foo1 

var foo2 = function () { // no function name i.e. anonymous function
   console.log('foo2'); 
} 
foo2(); // foo2 

A programming language is said to have first-class functions if a function can be treated the same way as any other variable in the language.

JavaScript has first-class functions.

Higher-Order Functions

Since JavaScript allows us to assign functions to variables, we can pass functions to other functions.

Functions that take functions as arguments are called higher-order functions.

A very common example of a higher-order function is setTimeout.

The following code shows how to use setTimeout function.

setTimeout(function () {
    console.log('2000 milliseconds have passed since this demo started'); 
}, 2000); 

If you run this application in Node.js, you will see the console.log message after two seconds and then the application will exit.

We used an anonymous function as the first argument to setTimeout.

This makes setTimeout a higher-order function.

We can create a function and passing that in.

function foo() {
    console.log('2000 milliseconds have passed since this demo started'); 
}
setTimeout(foo, 2000); 

Closures

If there is a function defined inside another function, the inner function has access to the variables declared in the outer function.

The variables in the outer function have been closed by the inner function.

The concept in itself is simple enough and fairly intuitive.

function outerFunction(arg) { 
    var variableInOuterFunction = arg;
    function myValue() { 
       console.log(variableInOuterFunction);
    }
    myValue(); 
} 
outerFunction('hello closure!'); // logs hello closure! 

The inner function can access the variables from the outer scope even after the outer function has returned.

Because the variables are still bound in the inner function and not dependent on the outer function.

function outerFunction(arg) {
    var variableInOuterFunction = arg;
    return function () {
       console.log(variableInOuterFunction); 
    } 
} 
var innerFunction = outerFunction('hello closure!'); 
innerFunction();

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