JavaScript Function 簡介

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. functionfoo() { // Do something. }

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

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; returnfunction() {        console.log( text );    }; }; var greeting = greet( "Rebecca", "Hello" ); greeting(); // "Hello, Rebecca"

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!

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"