First identify what is repeating: a script tag in the page, a network transfer, script initialization, or JavaScript code in multiple build outputs. These are different problems, and each needs a different fix. React can deduplicate certain external scripts; Vue recommends keeping script tags out of rendered templates; and Vue and Angular offer ways to load application modules on demand.
Find out what is repeating
“Loaded multiple times” can describe several distinct observations. Check the relevant layer before changing framework code:
- Duplicate script elements: Inspect the document or browser developer tools for multiple
<script>elements with the samesrc. - Repeated network transfers: In the Network panel, check whether the browser made more than one request for the resource. A repeated element and a repeated transfer are not automatically the same issue.
- Repeated execution or initialization: Look for multiple calls to setup code, event handlers, or a third-party library initializer. A single downloaded file can still be initialized more than once.
- Duplicate code in build outputs: Inspect the production build’s chunks or bundle analysis. Code appearing in multiple outputs is a build-splitting concern, not necessarily a duplicate script tag.
These checks separate DOM insertion, network activity, runtime behavior, and bundle composition. The framework options below address particular mechanisms; none should be assumed to fix all four.
React: deduplicate matching external scripts
React’s built-in browser <script> component has special handling when rendered with an external src and async={true}. React documents that it deduplicates matching source URLs: “React will de-duplicate scripts that have the same src, inserting only one of them into the DOM even if multiple components render it.” See the React <script> reference.
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<script async={true} src="https://example.com/library.js" />This is a same-src rule, not a general guarantee for different URLs or repeated application initialization. Ensure the components use the same source URL and the documented async prop if relying on this behavior.
Wait until the script has loaded
A component can commit before its script finishes loading. Code that depends on the global or API provided by that script should wait for the load event; React documents onLoad as an option. Do not treat rendering the script element as proof that its contents are already available.
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Account for the script’s lifetime
React’s documentation also notes that props are ignored after the script has rendered and that the script may remain in the DOM after the component that rendered it unmounts. If the library requires teardown, or should be initialized only once, manage that lifecycle explicitly rather than assuming unmounting removes the script or reverses its effects.
Vue: keep script elements out of rendered templates
Vue strongly discourages rendering <script> elements from templates or render functions because rendering should not introduce side effects. This guidance concerns script elements in Vue rendering; it does not mean Vue applications cannot use JavaScript modules or external scripts. See the Vue security guide.
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Load an asynchronous component with a dynamic import
Vue documents defineAsyncComponent with a dynamic import. Build tools such as Vite and webpack can use the dynamic import as a bundle split point, allowing the component module to be loaded separately when needed. See the Vue async components guide.
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import { defineAsyncComponent } from 'vue'
const SettingsPanel = defineAsyncComponent(() =>
import('./SettingsPanel.vue')
)Asynchronous loading can take time or fail, so provide appropriate loading and error UI for the component’s role in the application.
Angular: defer eligible code and verify the split
Angular’s @defer feature can place eligible components, directives, and pipes in a separate JavaScript file and load that code when needed. Angular generates dynamic imports for deferred dependencies, but does not guarantee a particular import order. See the Angular @defer guide.
Check imports and dependency type
Adding a defer block does not by itself prove that the intended code was split out of the main bundle. Angular identifies barrel imports as a common reason a component may remain in the main bundle and recommends importing directly from the component’s own file. The guide also says NgModule-based dependencies are not deferred. Check these constraints when the production output does not match expectations.
Confirm the production result
Inspect the built application and observe when the relevant chunk is requested at runtime. Verify that the code is fetched at the intended point and that the deferred UI has suitable loading and failure handling. Do not infer successful splitting solely from the presence of an @defer block.
Choose the fix that matches the problem
| Approach | What it addresses | When code is fetched | Important limits |
|---|---|---|---|
React async <script> |
Insertion of an external script element | When the script is rendered and loaded | React’s documented deduplication requires async={true} and matching src values; it does not ensure one-time initialization. |
| Vue async component with dynamic import | Loading a component module as a split point | When the async component is needed | Handle loading and errors; bundle behavior depends on the build tool and output. |
Angular @defer |
Deferring eligible component, directive, and pipe code | When the defer trigger calls for it | Barrel imports can keep code in the main bundle; NgModule-based dependencies are not deferred. |
These approaches do not automatically prevent repeated runtime setup. If a library is downloaded once but behaves as though it is active multiple times, find and control the initialization path separately. If the concern is code repeated across output files, focus on imports and the production bundle rather than adding another script-level guard.
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