Modular programming breaks down large applications into smaller, easy-to-manage code blocks. Module-based coding simplifies maintenance and improves code reusability. However, the dependencies between management modules are a major problem that developers face throughout the application development process. RequireJS is one of the most popular frameworks for managing dependencies between modules. This tutorial explores the requirements of modular code and shows how RequireJS can help.

Key Points

• RequireJS is a popular framework for managing dependencies between JavaScript modules, which improves the speed and quality of your code, especially in large projects.
• RequireJS uses asynchronous module loading (AMD) to load files, which allows scripts to load modules and their dependencies in a non-blocking manner.
• In RequireJS, all code is wrapped in require() or define() functions. The require() function is used for functions that are executed immediately, while the define() function is used to define modules that can be used in multiple locations.
• RequireJS improves code quality by promoting modularity and separation of concerns, reduces the risk of naming conflicts by keeping global scope neat and provides a powerful error handling mechanism.

Load JavaScript file

Large applications usually require many JavaScript files. Usually, they use <p> <code>credits.js

Here, the initialization is done before loading

. This will result in an error as shown below. This example only requires three JavaScript files. In a larger project, things can easily get out of control. This is where RequireJS comes into play.

RequireJS Introduction

RequireJS is a well-known JavaScript module and file loader supported by the latest versions of popular browsers. In RequireJS, we separate the code into modules, each of which handles a single responsibility. In addition, dependencies need to be configured when loading the file. Let's start with downloading RequireJS. After the download is complete, copy the file to your project folder. Let's assume that the directory structure of the project is now similar to the following figure:

scriptsmain.js

<script> 標(biāo)簽逐個(gè)加載。此外，每個(gè)文件都可能依賴于其他文件。最常見的例子是 jQuery 插件，它們都依賴于核心 jQuery 庫。因此，必須在加載任何 jQuery 插件之前加載 jQuery。讓我們來看一個(gè)在實(shí)際應(yīng)用程序中加載 JavaScript 文件的簡單示例。假設(shè)我們有以下三個(gè) JavaScript 文件： <p><code>purchase.js &lt;pre class='brush:php;toolbar:false;'&gt;function purchaseProduct(){ console.log(&amp;quot;Function : purchaseProduct&amp;quot;); var credits = getCredits(); if(credits &amp;gt; 0){ reserveProduct(); return true; } return false; }&lt;/pre&gt; &lt;p&gt;&lt;code&gt;products.js</code></p> <pre class='brush:php;toolbar:false;'>function reserveProduct(){ console.log(&quot;Function : reserveProduct&quot;); return true; }</pre> <p><code>credits.js</code></p> <pre class='brush:php;toolbar:false;'>function getCredits(){ console.log(&quot;Function : getCredits&quot;); var credits = &quot;100&quot;; return credits; }</pre> <p>在這個(gè)例子中，我們?cè)噲D購買一個(gè)產(chǎn)品。首先，它檢查是否有足夠的積分可以購買產(chǎn)品。然后，在驗(yàn)證積分后，它預(yù)訂產(chǎn)品。另一個(gè)腳本 <code>main.js</code> 通過調(diào)用 <code>purchaseProduct()</code> 來初始化代碼，如下所示：</p> <pre class='brush:php;toolbar:false;'>var result = purchaseProduct();</pre> <p><strong>可能出錯(cuò)的地方？</strong></p> <p>在這個(gè)例子中，<code>purchase.js</code> 依賴于 <code>credits.js</code> 和 <code>products.js</code>。因此，在調(diào)用 <code>purchaseProduct()</code> 之前需要加載這些文件。那么，如果我們按以下順序包含 JavaScript 文件會(huì)發(fā)生什么情況呢？</p> <pre class='brush:php;toolbar:false;'>&lt;script src=&quot;products.js&quot;&gt;&lt;/script&gt;All JavaScript files (including RequireJS files) are located in the &lt;script src=&quot;purchase.js&quot;&gt;&lt;/script&gt; folder. &lt;script src=&quot;main.js&quot;&gt;&lt;/script&gt; Files are used for initialization, and other files contain application logic. Let's see how to include scripts in HTML files. &lt;script src=&quot;credits.js&quot;&gt;&lt;/script&gt;&lt;pre class=&quot;brush:php;toolbar:false&quot;&gt;&lt;code class=&quot;html&quot;&gt;&lt;??&gt;</pre> <p>This is the only code you need to include the file using RequireJS. You may be wondering what's going on with other files and how they are included. The <code>data-main</code> attribute defines the initialization point of the application. In this case, it is <code>main.js</code>. RequireJS uses <code>main.js</code> to find other scripts and dependencies. In this case, all files are in the same folder. Using logic, you can move files to any folder you like. Now, let's take a look at <code>main.js</code>. </p> <pre class='brush:php;toolbar:false;'>require([&quot;purchase&quot;],function(purchase){ purchase.purchaseProduct(); });</pre> <p>In RequireJS, all code is wrapped in <code>require()</code> or <code>define()</code> functions. The first argument of these functions specifies the dependency. In the previous example, the initialization depends on <code>purchase.js</code> because it defines <code>purchaseProduct()</code>. Please note that the file extension has been omitted. This is because RequireJS only considers <code>.js</code> files. The second argument to <code>require()</code> is an anonymous function that accepts an object that calls the functions contained in the dependent file. In this case, we have only one dependency. Multiple dependencies can be loaded using the following syntax: </p> <pre class='brush:php;toolbar:false;'>require([&quot;a&quot;,&quot;b&quot;,&quot;c&quot;],function(a,b,c){ });</pre> <p><strong>Create an application with RequireJS</strong></p> <p>In this section, we will convert the pure JavaScript example discussed in the previous section to RequireJS. We've covered <code>main.js</code>, so let's go on to discuss other documents. <code>purchase.js</code></p> <pre class='brush:php;toolbar:false;'>define([&quot;credits&quot;,&quot;products&quot;], function(credits,products) { console.log(&quot;Function : purchaseProduct&quot;); return { purchaseProduct: function() { var credit = credits.getCredits(); if(credit &gt; 0){ products.reserveProduct(); return true; } return false; } } });</pre> <p>First of all, we declare that the purchase function depends on <code>credits</code> and <code>products</code>. In the <code>return</code> statement, we can define the functions of each module. Here we have called the <code>getCredits()</code> and <code>reserveProduct()</code> functions on the passed object. <code>product.js</code> is similar to <code>credits.js</code>, as shown below. <code>products.js</code></p> <pre class='brush:php;toolbar:false;'>define(function(products) { return { reserveProduct: function() { console.log(&quot;Function : reserveProduct&quot;); return true; } } });</pre> <p><code>credits.js</code></p> <pre class='brush:php;toolbar:false;'>define(function() { console.log(&quot;Function : getCredits&quot;); return { getCredits: function() { var credits = &quot;100&quot;; return credits; } } });</pre> <p> Both files are configured as standalone modules—which means they do not depend on anything. The important thing to note is that <code>define()</code> is used instead of <code>require()</code>. Selecting <code>require()</code> or <code>define()</code> depends on the structure of the code and will be discussed in the next section. </p> <p><strong>Use <code>require()</code> with <code>define()</code></strong> </p>I mentioned earlier that we can use <p> and <code>require()</code> to load dependencies. Understanding the difference between these two functions is essential for managing dependencies. The <code>define()</code> function is used to run the function executed immediately, while the <code>require()</code> function is used to define modules that can be used in multiple locations. In our example, we need to run the <code>define()</code> function immediately. Therefore, <code>purchaseProduct()</code> is used in <code>require()</code>. However, other files are reusable modules, so use <code>main.js</code>. <code>define()</code> </p><p>Why RequireJS is so important<strong></strong><p>In pure JavaScript examples, an error is generated due to the incorrect loading order of files. Now, delete the <code>credits.js</code> file in the RequireJS example and see how it works. The following figure shows the output of the browser check tool. </p> <p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/174036404874237.jpg" class="lazy" alt="Understanding RequireJS for Effective JavaScript Module Loading " /></p> <p>The difference here is that no code is executed in the RequireJS example. We can confirm this because nothing is printed on the console. In the pure JavaScript example, we print some output on the console before generating the error. RequireJS waits before loading all dependent modules before executing the function. If any module is lost, it will not execute any code. This helps us maintain the integrity of our data. </p> <p><strong>Sequence of managing dependency files</strong></p> <p>RequireJS uses asynchronous module loading (AMD) to load files. Each dependent module will start loading with asynchronous requests in the given order. Even with the file order taken into account, we cannot guarantee that the first file will be loaded before the second file due to the asynchronous nature. Therefore, RequireJS allows us to use the shim configuration to define the sequence of files that need to be loaded in the correct order. Let's see how to create configuration options in RequireJS. </p> <pre class='brush:php;toolbar:false;'>&lt;??&gt;</pre> <p>RequireJS allows us to provide configuration options using the <code>config()</code> function. It accepts a parameter named <code>shim</code> which we can use to define a mandatory sequence of dependencies. You can find a complete configuration guide in the RequireJS API documentation. </p> <pre class='brush:php;toolbar:false;'>require([&quot;purchase&quot;],function(purchase){ purchase.purchaseProduct(); });</pre> <p> Under normal circumstances, these four files will start loading in the given order. Here, <code>source2 depends on <code>source1. Therefore, once <code>source1 has finished loading, <code>source2 will consider all dependencies to be loaded. However, <code>dependency1 and <code>dependency2 may still be loading. Using shim configuration, dependencies must be loaded before <code>source1. Therefore, no error is generated. <p><strong>Conclusion <p>I hope this tutorial will help you get started with RequireJS. While it looks simple, it is really powerful in managing dependencies in large JavaScript applications. This tutorial alone is not enough to cover all aspects of RequireJS, so I hope you can learn all the advanced configurations and techniques using the official website. <p><strong> (The following is a pseudo-original creation of the FAQs part in the original text, maintaining the original meaning, and adjusting and rewriting the sentences) <p><strong>FAQs about JavaScript module loading using RequireJS <p><strong>What is the main use of RequireJS in JavaScript? <p>RequireJS is a JavaScript file and module loader that is optimized for browser use, but is also suitable for other JavaScript environments. The main purpose of using RequireJS is to improve the speed and quality of your code. It helps you manage dependencies between code modules and load scripts in an efficient way. This is especially useful in large projects where single scripts can become complex. RequireJS also helps keep the global scope clean by reducing the use of global variables. <p><strong>How to deal with dependencies? <p>RequireJS handles dependencies through a mechanism called asynchronous module definition (AMD). This allows the script to load modules and their dependencies in a non-blocking manner. When you define a module, you specify its dependencies, RequireJS ensures that these dependencies are loaded before the module itself. This way, you can ensure that all necessary code is available when executing the module. <p><strong>Can RequireJS be used with Node.js? <p>Yes, RequireJS can be used with Node.js, although it is more commonly used in browsers. When used with Node.js, RequireJS allows you to organize server-side JavaScript code into modules just like in your browser. However, Node.js has its own module system (CommonJS), so using RequireJS is less common. <p><strong>How to improve code quality? <p>RequireJS improves code quality by promoting modularity and separation of concerns. By organizing the code into modules, each with its specific functionality, you can write code that is easier to maintain and test. It also reduces the risk of naming conflicts by keeping the global scope neat. <p><strong>What is the difference between RequireJS and CommonJS? <p>RequireJS and CommonJS are both JavaScript module systems, but they have some key differences. RequireJS uses the Asynchronous Module Definition (AMD) format that is designed to load modules and their dependencies asynchronously in the browser. On the other hand, CommonJS used by Node.js synchronizes the loading of modules, which is more suitable for server-side environments. <p><strong>How to define modules in RequireJS? <p>In RequireJS, you can use the <code>define function to define modules. This function takes two parameters: a dependency array and a factory function. Once all dependencies are loaded, the factory function is called and the value of the module should be returned. <p><strong>How to load modules in RequireJS? <p>To load a module in RequireJS, you can use the <code>require function. This function accepts two parameters: a dependency array and a callback function. Once all dependencies are loaded, the callback function is called. <p><strong>Can I use RequireJS with other JavaScript libraries? <p>Yes, RequireJS can be used with other JavaScript libraries such as jQuery, Backbone, and Angular. It can load these libraries as modules and manage their dependencies. <p><strong>How to handle errors? <p>RequireJS has a powerful error handling mechanism. If the script fails to load, RequireJS will trigger an error event. You can listen for this event and handle errors in your code appropriately. <p><strong>Can I use RequireJS for production? <p>Yes, RequireJS is suitable for development and production environments. For production environments, RequireJS provides an optimization tool that combines and compresses your JavaScript files to improve loading time. </script>

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