JavaScript modular Programming (II): AMD specifications

The first part of this series introduces the basic wording of the JavaScript module, and today describes how to use the module in a canonical manner.

VII. Specification of modules

First think about why the module is important?

Because of the module, we can more easily use other people's code, want what function, load what module.

However, this has a premise, that is, we must write the module in the same way, otherwise you have your writing, I have my writing, it is not messy set! This is even more important considering that the JavaScript module does not yet have an official specification.

Currently, there are two types of JavaScript module specifications available: Commonjs and AMD. I mainly introduce AMD, but start with Commonjs.

Eight, CommonJS

In 2009, American programmer Ryan Dahl created the node. JS project to use the JavaScript language for server-side programming.

This symbol "JavaScript modular programming" was formally born. Because frankly speaking, in the browser environment, no module is not particularly big problem, after all, the complexity of the Web-page program is limited, but on the server side, must have modules, and the operating system and other applications to interact with, otherwise it is impossible to program.

Node. JS's module system is implemented with reference to the COMMONJS specification. In Commonjs, there is a global method require () that is used to load the module. Assuming that a mathematical module is math.js, it can be loaded as follows.

var math = require (' math ');

Then, you can invoke the method provided by the module:

var math = require (' math '); Math.add (//  5

Since this series is primarily for browser programming and does not involve node. js, there is no more introduction to COMMONJS. As long as we know here, require () is used to load the module on the line.

Nine, the browser environment

With the server-side module, it is natural for everyone to want the client module. And it is best to be compatible, a module without modification, both server and browser can be run.

However, due to a significant limitation, the COMMONJS specification does not apply to the browser environment. or the previous section of the code, if run in the browser, there will be a big problem, can you see?

var math = require (' math '); Math.add (2, 3);

The second line, Math.add (2, 3), runs after the first line of require (' math '), so it must wait until the math.js load is complete. That is, if the load time is long, the entire application will stop there and so on.

This is not a problem on the server side, because all the modules are stored on the local hard disk, can be loaded synchronously, waiting time is the hard disk read time. However, for the browser, this is a big problem, because the modules are placed on the server side, the waiting time depends on the speed of the network, it may take a long time, the browser is in "Suspended animation" status.

As a result, the browser-side module cannot be "synchronous-loaded" (synchronous) and can only take "asynchronous load" (asynchronous). This is the background to the birth of the AMD specification.

X. AMD

AMD is the abbreviation for "Asynchronous module definition", meaning "async module definition". It loads the module asynchronously, and the module's load does not affect the execution of the statement behind it. All statements that rely on this module are defined in a callback function that will not run until the load is complete.

AMD also uses the Require () statement to load the module, but unlike COMMONJS, it requires two parameters:

Require ([module], callback);

The first parameter, [module], is an array in which the member is the module to be loaded, and the second parameter, callback, is the callback function after the load succeeds. If you rewrite the previous code in AMD form, this is the following:

function　　(math) {Math.add (2, 3); });

Math.add () is not synchronized with the math module, and the browser does not take place in animation. So it's clear that AMD is better suited for a browser environment.

Currently, there are two main JavaScript libraries that implement the AMD specification: Require.js and Curl.js. The third part of this series will introduce require.js to further explain AMD's usage and how to put modular programming into combat.

JavaScript modular Programming (II): AMD specifications