Posted in: JavaScript 101

Functions

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, two of which are shown below:

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// Function declaration.
function foo() {
// Do something.
}
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// Named function expression.
var foo = function() {
// Do something.
};

link Using Functions

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// A simple function.
var greet = function( person, greeting ) {
var text = greeting + ", " + person;
console.log( text );
};
greet( "Rebecca", "Hello" ); // "Hello, Rebecca"
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// A function that returns a value.
var greet = function( person, greeting ) {
var text = greeting + ", " + person;
return text;
};
console.log( greet( "Rebecca", "Hello" ) ); // "Hello, Rebecca"
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// A function that returns another function.
var greet = function( person, greeting ) {
var text = greeting + ", " + person;
return function() {
console.log( text );
};
};
var greeting = greet( "Rebecca", "Hello" );
greeting(); // "Hello, Rebecca"

link Immediately-Invoked Function Expression (IIFE)

A common pattern in JavaScript is the immediately-invoked function expression. 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 code – no variables declared inside of the function are visible outside of it.

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// An immediately-invoked function expression.
(function() {
var foo = "Hello world";
})();
console.log( foo ); // undefined!

link 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.

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// Passing an anonymous function as an argument.
var myFn = function( fn ) {
var result = fn();
console.log( result );
};
// Logs "hello world"
myFn( function() {
return "hello world";
});
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// Passing a named function as an argument
var myFn = function( fn ) {
var result = fn();
console.log( result );
};
var myOtherFn = function() {
return "hello world";
};
myFn( myOtherFn ); // "hello world"