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Last updated on Mar 19, 2025
•10 mins read
Last updated on Mar 19, 2025
•10 mins read
Preloading is a key technique in modern web development that speeds up application performance by reducing load times and user friction. By telling the browser to fetch and prepare the JavaScript modules before they are needed, we can optimize resource loading and execution. In React, the preloadModule
function helps us do that.
This blog will walk you through preloading modules in React to speed up performance.
The preloadModule
function in React serves as a hint to the browser to start downloading a specific ECMAScript Module (ESM) in anticipation of its future use. This proactive approach ensures that the module and its dependencies are fetched and prepared ahead of time, reducing delays during execution. The function is part of the react-dom
package and is utilized as follows:
1import { preloadModule } from 'react-dom'; 2 3function AppRoot() { 4 preloadModule("https://example.com/module.js", { as: "script" }); 5 // Additional component logic 6}
• href: A string representing the URL of the module to be preloaded.
• options: An object containing:
'script'
.'anonymous'
or 'use-credentials'
).Returns: The function does not return any value.
Preloading modules offers several advantages:
If a component or its children will require a specific module, you can preload it during the rendering phase:
1import { preloadModule } from 'react-dom'; 2 3function AppRoot() { 4 preloadModule("https://example.com/module.js", { as: "script" }); 5 return ( 6 // Component JSX 7 ); 8}
In this example, the preloadModule
function is called during the rendering of AppRoot
, ensuring that the specified module is fetched and prepared before any of its functionalities are invoked.
To preload a module in response to user interactions, such as clicking a button, you can implement the following approach:
1import { preloadModule } from 'react-dom'; 2 3function CallToAction() { 4 const onClick = () => { 5 preloadModule("https://example.com/module.js", { as: "script" }); 6 // Additional logic 7 }; 8 9 return ( 10 <button onClick={onClick}>Start Action</button> 11 ); 12}
Here, the preloadModule
function is invoked within the onClick
event handler, initiating the module's download when the user interacts with the button. This strategy ensures that the module is ready for use by the time it's needed in the application flow.
In module-based development, applications are often divided into smaller, reusable pieces known as modules. These modules frequently depend on one another, creating a network of module dependencies. When a browser encounters a module, it must fetch and parse it to identify its dependencies, which are then fetched and parsed in turn. This sequential process can introduce delays, especially if there are deep dependency trees.
Preloading modules addresses this challenge by informing the browser about all necessary modules upfront. By using preloadModule
or <link rel="modulepreload">
, developers can instruct the browser to fetch and compile modules and their dependencies ahead of time, minimizing delays associated with module loading.
<link rel="modulepreload">
for Module PreloadingBeyond React's preloadModule
, the HTML <link rel="modulepreload">
element offers a declarative approach to preloading JavaScript modules. This method allows developers to specify modules to be preloaded directly within the HTML document, ensuring that both the main module and its dependencies are fetched and compiled before execution.
Example:
1<head> 2 <link rel="modulepreload" href="https://example.com/module.js"> 3</head> 4<body> 5 <script type="module" src="https://example.com/module.js"></script> 6</body>
In this setup, the <link rel="modulepreload">
element instructs the browser to preload the specified module and its dependencies. When the <script type="module">
is encountered, the module is already in the module map, ready for execution.
Definition: Lazy loading is a strategy that delays the loading of non-essential resources until they are required. For instance, images or modules are loaded only when they come into the user's viewport or when a specific action is taken. This approach reduces the initial load time of a web page, leading to faster rendering and improved performance.
Use Cases:
Benefits:
Considerations:
Definition: Preloading is a technique where resources anticipated to be needed soon are loaded in advance, typically during idle times. This proactive approach ensures that critical resources are ready before the user requests them, thereby reducing wait times during interactions.
Use Cases:
Benefits:
Considerations:
The decision to use preloading or lazy loading depends on the specific requirements and characteristics of the application:
In practice, a hybrid approach is often employed, leveraging both techniques to optimize performance. For example, critical resources can be preloaded to ensure they're ready when needed, while non-essential resources are lazy-loaded to conserve bandwidth and improve initial load times.
React supports lazy loading through the React.lazy()
function and the Suspense
component. This allows components to be loaded on demand, reducing the initial bundle size.
1import React, { Suspense } from 'react'; 2 3const LazyComponent = React.lazy(() => import('./LazyComponent')); 4 5function App() { 6 return ( 7 <div> 8 <Suspense fallback={<div>Loading...</div>}> 9 <LazyComponent /> 10 </Suspense> 11 </div> 12 ); 13}
While React doesn't have a built-in preloading mechanism, developers can use standard HTML <link>
tags with rel="preload"
in the public/index.html
file or utilize third-party libraries to manage preloading of critical resources.
1<link rel="preload" href="/static/js/main.chunk.js" as="script">
It's essential to delve deeper into the mechanics and best practices associated with <link rel="modulepreload">
. This HTML attribute provides a declarative approach to preemptively fetch module scripts, parse and compile them, and store them in the document's module map for later execution.
<link rel="modulepreload">
To utilize <link rel="modulepreload">
, you can include it within the <head>
section of your HTML document:
1<head> 2 <link rel="modulepreload" href="https://example.com/module.js"> 3</head>
This approach informs the browser to fetch and prepare the specified module ahead of time, ensuring that when the module is needed, it's ready for execution. Unlike traditional preloading methods, modulepreload
not only downloads the module but also parses and compiles it, storing the results in the module map.
A critical aspect of module-based development is managing module dependencies. When a browser encounters a module, it must fetch and parse it to identify its dependencies, which are then fetched and parsed in turn. This sequential process can introduce delays, especially if there are deep dependency trees. Preloading modules addresses this challenge by informing the browser about all necessary modules upfront. By using <link rel="modulepreload">
, developers can instruct the browser to fetch and compile modules and their dependencies ahead of time, minimizing delays associated with module loading.
While modulepreload
is a powerful tool, it's essential to consider browser compatibility. This feature is available across the latest devices and browser versions. However, older browsers may not support it, making it a progressive enhancement. In such cases, the application will still function, but without the performance benefits of preloading.
Additionally, when using modulepreload
, the fetch request mode is always cors
, and the crossorigin
attribute determines the request's credentials mode. If crossorigin
is set to anonymous
or left empty (the default), the credentials mode is same-origin
, meaning user credentials like cookies are only sent for same-origin requests. If crossorigin
is set to use-credentials
, the credentials mode is include
, sending user credentials for both same-origin and cross-origin requests.
Consider a scenario where your main module, main.js
, imports two dependency modules, canvas.js
and square.js
. Traditionally, the browser would fetch main.js
, parse it, and then sequentially fetch its dependencies, leading to potential delays. By implementing modulepreload
, you can instruct the browser to fetch all necessary modules upfront:
1<head> 2 <link rel="modulepreload" href="modules/square.js"> 3 <link rel="modulepreload" href="modules/canvas.js"> 4 <link rel="modulepreload" href="main.js"> 5</head>
This approach ensures that all modules are fetched and compiled in parallel, reducing the overall load time and enhancing the user experience.
Using module preloading methods like React’s preloadModule
function and the HTML <link rel="modulepreload">
tag helps to load modules faster. This improves app performance and makes it smoother. By setting it up right, developers can make their apps more responsive and reliable.
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