JavaScript Function

The definition expression defines a JavaScript function, and the value of such an expression is the newly defined function. In a sense, a function definition expression is a “function literal” in the same way that an object initializer is an “object literal.” A function definition expression typically consists of the keyword function followed by a comma-separated list of zero or more identifiers (the parameter names) in parentheses and a block of JavaScript code (the function body) in curly braces.

A lot of the values provided in the default environment have the type function. A function is a piece of program wrapped in a value. Such values can be applied in order to run the wrapped program.

JavaScript function

Functions contain blocks of code that need to be executed repeatedly. Functions can take zero or more arguments, and can optionally return a value.

Functions can be created in a variety of ways:

Function Declaration

function foo () { /* Technosap */ }

Named Function Expression

var foo = function () { /* Technosap */ }

I prefer the named function expression method of setting a function’s name, for some rather in-depth and technical reasons. You are likely to see both methods used in others’ JavaScript code.

Using JavaScript Function

A simple function

var greet = function ( person , greeting ) {
var text = greeting + ’, ’ + person ;
console . log ( text );
};
greet ( ’ Rebecca ’ , ’ Hello ’);

A function that returns a value

var greet = function ( person , greeting ) {
var text = greeting + ’, ’ + person ;
return text ;
};

console . log ( g r e e t ( ’ Rebecca ’ , ’ Technosap ’));

function that returns another function

var greet = function ( person , greeting ) {
var text = greeting + ’, ’ + person ;
return function () { console. log ( text ); };
};
var greeting = greet( ’ Rebecca ’ , ’ Technosap ’);
greeting ();

Self-Executing Anonymous Functions

A common pattern in JavaScript is the self-executing anonymous function. This pattern creates a function expression and then immediately executes the function. This pattern is extremely useful for cases where you want to avoid polluting the global namespace with your code — no variables declared inside of the function are visible outside of it.

A self-executing anonymous function

( function (){
var foo = ’ Hello world ’;
} ) ( ) ;
console . log ( foo ); // undefined !

Functions as Arguments

In JavaScript, functions are “first-class citizens” — they can be assigned to variables or passed to other functions as arguments. Passing functions as arguments is an extremely common idiom in jQuery.

Passing an anonymous function as an argument

var myFn = function ( fn ) {
var result = fn ();
console . log ( result );
};
myFn ( function () { return ’ hello world ’; }); // logs ’ hello world ’

Passing a named function as an argument

var myFn = function ( fn ) {
var result = fn ();
console. log ( result );
};
var my O the r F n = function () {
return ’ hello world ’;
};
myFn ( my O therFn ); // logs ’ hello world ’

A JavaScript function definition expression can also include a name for the function. Functions can also be defined using a function statement rather than a function expression.