JavaScript modular programming (1): How to write modules

As websites gradually become "Internet applications", the Javascript codes embedded in web pages are becoming larger and more complex.

Web pages are becoming more and more like desktop programs, requiring a team's division of labor and collaboration, progress management, unit testing, etc... Developers have to use software engineering methods to manage web pages business logic.

JavaScript modularization Programming has become an urgent need. Ideally, developers only need to implement the core business logic, and other modules can be loaded by others.

However, Javascript is not a modular programming language. It does not support "classes", let alone "modules". (The sixth edition of the ECMAScript standard, which is being formulated, will officially support "classes" and "modules", but it will take a long time to be put into practical use.)

The Javascript community has made a lot of efforts to improve the existing In the running environment, the effect of "module" is realized. This article summarizes the current best practices of "Javascript modular programming" and explains how to put them into practice. Although this is not an introductory tutorial, you can understand it as long as you have a little understanding of the basic syntax of Javascript.

1. Original writing

A module is a set of methods to implement specific functions.

As long as different functions (and variables that record status) are simply put together, it is considered a module.

function m1(){
　　//...
}
function m2(){
　　//...
}

The above functions m1() and m2() form a module. When using it, just call it directly.

The disadvantages of this approach are obvious: it "pollutes" global variables, there is no guarantee that variable names will not conflict with other modules, and there is no direct relationship between module members.

2. Object writing method

In order to solve the above shortcomings, the module can be written as an object, and all module members are placed in this object.

　　var module1 = new Object({
　　　　_count : 0,
　　　　m1 : function (){
　　　　　　//...
　　　　},
　　　　m2 : function (){
　　　　　　//...
　　　　}
　　});

The above functions m1() and m2() are encapsulated in the module1 object. When used, the properties of this object are called.

module1.m1();

However, this way of writing will expose all module members, and the internal state can be rewritten externally. For example, external code can directly change the value of an internal counter.

module1._count = 5;

3. How to write an immediately executed function

Using "Immediately-Invoked Function Expression (IIFE)" can achieve the purpose of not exposing private members.

　　var module1 = (function(){
　　　　var _count = 0;
　　　　var m1 = function(){
　　　　　　//...
　　　　};
　　　　var m2 = function(){
　　　　　　//...
　　　　};
　　　　return {
　　　　　　m1 : m1,
　　　　　　m2 : m2
　　　　};
　　})();

Using the above writing method, external code cannot read the internal _count variable.

console.info(module1._count); //undefined

module1 is the basic writing method of Javascript module. Next, we will process this writing method.

4. Amplification mode

If a module is very large and must be divided into several parts, or one module needs to inherit another module, then it is necessary to use the "amplification mode" (augmentation) .

　　var module1 = (function (mod){
　　　　mod.m3 = function () {
　　　　　　//...
　　　　};
　　　　return mod;
　　})(module1);

The above code adds a new method m3() to the module1 module, and then returns the new module1 module.

5. Loose augmentation mode

In the browser environment, each part of the module is usually obtained from the Internet, and sometimes it is impossible to know which part will be loaded first. If the writing method in the previous section is adopted, the first executed part may load a non-existent empty object, in which case the "widening mode" must be used.

　　var module1 = ( function (mod){
　　　　//...
　　　　return mod;
　　})(window.module1 || {});

Compared with "zoom-in mode", "wide magnification mode" means that the parameters of "immediate execution function" can be empty objects.

6. Input global variables

Independence is an important feature of the module. It is best not to directly interact with other parts of the program inside the module.

In order to call global variables inside a module, other variables must be explicitly entered into the module.

　　var module1 = (function ($, YAHOO) {
　　　　//...
　　})(jQuery, YAHOO);

The module1 module above needs to use the jQuery library and the YUI library, so these two libraries (actually two modules) are entered into module1 as parameters. In addition to ensuring the independence of the modules, this also makes the dependencies between modules obvious. For more discussion in this regard, see Ben Cherry's famous article "JavaScript Module Pattern: In-Depth".

The second part of this series will discuss how to organize different modules and manage dependencies between modules in the browser environment.

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