Callback Functions in JavaScript

Original: http://javascriptissexy.com/

Translation: http://blog.csdn.net/luoweifu/article/details/41466537

"Recommended to read the original text, the following is a personal study notes, some knowledge points to add, do not like to spray"

In Javascrip, a function is a built-in class object, which means that it is a type of object that can be used for the management of built-in objects just like other objects of the string, Array, number, and object classes. Because the function is actually an object, it can be "stored in a variable, passed to (another) function by argument, created inside a function, and returns the result value from the function." Because function is a built-in object, we can pass it as an argument to another function, defer execution to the function, or even return it after execution. This is the essence of using a callback function in JavaScript. The remainder of this article will fully learn the JavaScript callback function. The callback function may be the most widely used functional programming technique in JavaScript, and perhaps just a small piece of JavaScript or jquery code will leave a mystery to the developer, and reading this article may help you eliminate the mystery.

The callback function comes from a well-known programming paradigm- functional programming , at the basic level, where functional programming specifies the parameters of a function. Functional programming Although the current scope of use has become smaller, but it has been "professional smart" programmers as a kind of difficult to understand technology, previously, and the future will be so.

Fortunately, functional programming has been elaborated by the General people like you and me can also understand and use. One of the main techniques of functional programming is the callback function, which you will soon read that implementing a callback function is as simple as passing a generic parameter variable. This technology is so simple that I wonder why it is often included in the high-level topic of JavaScript.

What is a callback or advanced function?

A callback function is considered an advanced function, an advanced function that is passed as a parameter to another function (called "otherfunction"), and the callback function is called (or executed) within the otherfunction. The essence of a callback function is a pattern (a pattern that solves common problems), so a callback function is also called a callback pattern.

Consider the following callback functions that are commonly used in jquery:

// Note The item in the click Method's parameter is a function and not a variable. // The item is a callback function$ ("#btn_1"). Click (function() {  alert ("BTN 1 Clicked" ) );});

As seen in the previous example, we pass a function to the parameter of the click Method, and the click Method invokes (or executes) the callback function we pass to it. This example gives a typical way to use the callback function in JavaScript and is widely used in jquery.

Savor a typical example of another basic javascript:

var friends = ["Mike", "Stacy", "Andy", "Rick"];friends.foreach (function//  1. Mike, 2. Stacy, 3. Andy, 4. Rick});

Once again, we passed an anonymous function in the same way (a function without a function name) to the Foreach method as a foreach parameter.

"Supplemental foreach"

The Oreach () method is used to invoke each element of the array and pass the element to the callback function.

So far, we have passed an anonymous function as a parameter to another function or method. Before looking at other more complex callback functions, let's take a look at how callbacks work and implement a callback function of our own.

Callback Functions in JavaScript