Nomodule is a boolean property of the <script> tag, used to be compatible with old browsers that do not support ES Modules. Its function is: the browser that supports module ignores scripts with nomodule, and loads it if it is not supported. This provides different code paths for different browsers. For example: <script type="module" src="modern.js"></script>. The actual usage scenarios include modular development and fallback packaged versions, lazy loading of polyfills. Note when using it: if you do not need to be compatible with old browsers, you do not need to use it, ensure that fallback code is compatible, test fallback validity, and avoid script conflicts.
In modern web development, the nomodule
property of the <script></script>
tag is actually a very practical but easily overlooked feature. It is not used to "disable" modules, but to be compatible with old browsers that do not support JavaScript modules ( type="module"
).

Simply put: if you use ES Modules and want old browsers to run code, nomodule
comes in handy .
What is nomodule
?
nomodule
is a boolean property of the <script></script>
tag. When the browser sees this property:

- If it supports modules (that is,
type="module"
), scripts withnomodule
will be ignored . - If it does not support modules , the script will be loaded and executed.
This means that you can provide different code paths for modern and old browsers respectively.
For example:

<script type="module" src="modern.js"></script> <script nomodule src="fallback.js"></script>
In the above code:
- The browser that supports modules will load
modern.js
and skipfallback.js
- Browsers that do not support modules will load
fallback.js
, skipmodern.js
This is very useful for gradual enhancement.
Why do you need nomodule
?
Although most mainstream browsers now support ES Modules, some old devices or company intranets may still use IE or older versions of Safari. These environments do not recognize type="module"
, so you cannot directly load modular JS to them.
If you do not process it, just send modular code to these browsers, and the result will be that JS errors and function failures.
At this time, nomodule
provides a clean way for you:
- Send lighter and more standard module code to new browsers
- Send packaged compatibility code to old browsers (such as bundle built through Babel Webpack)
This will not only enjoy the advantages of modern features without losing compatibility.
Actual use scenarios
Scenario 1: Modular development rollback packaged version
This is the most common usage. You use a modular structure during the development stage and output two versions when building:
- A modern version for a browser that supports modules
- A packaged traditional version for old browsers
Then the HTML is written as follows:
<script type="module" src="/assets/main.js"></script> <script nomodule src="/assets/legacy-bundle.js"></script>
Scene 2: Lazy loading polyfill
You can also use nomodule
to automatically introduce polyfill, such as:
<script type="module"> // Here is modern code</script> <script nomodule src="https://polyfill.io/v3/polyfill.min.js"></script>
In this way, only browsers that do not support modules will load polyfills to reduce unnecessary requests.
Notes on using
- Don't abuse
nomodule
: If you don't need to be compatible with old browsers, there is no need to use it. - Make sure the fallback script is indeed compatible with the old environment : for example, do not use the
import
syntax inside. - Testing is important : Test whether fallback works properly in a real old device or browser emulator.
- Avoid conflicts caused by loading multiple scripts at the same time : For example, both scripts have the same variable name defined, which may cause problems.
Overall, nomodule
is a simple but smart design that helps us find a balance between modern development and backward compatibility. Although not used every day, it can really solve the problem when needed.
Basically that's it.
The above is the detailed content of The `nomodule` Attribute for JavaScript Modules. For more information, please follow other related articles on the PHP Chinese website!

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