ASP.NET Core Blazor JavaScript with static server-side rendering (static SSR)

This article explains how to load JavaScript (JS) in a Blazor Web App with static server-side rendering (static SSR) and enhanced navigation.

Some apps depend on JS to perform initialization tasks that are specific to each page. When using Blazor's enhanced navigation feature, which allows the user to avoid reloading the entire page, page-specific JS may not be executed again as expected each time an enhanced page navigation occurs.

To avoid this problem, we don't recommended relying on page-specific <script> elements placed outside of the layout file applied to the component. Instead, scripts should register an afterWebStarted JS initializer to perform initialization logic and use an event listener (blazor.addEventListener("enhancedload", callback)) to listen for page updates caused by enhanced navigation.

The following example demonstrates one way to configure JS code to run when a statically-rendered page with enhanced navigation is initially loaded or updated.

The following PageWithScript component example is a component in the app that requires scripts to run with static SSR and enhanced navigation. The following component example includes a PageScript component from a Razor class library (RCL) that's added to the solution later in this article.

Components/Pages/PageWithScript.razor:

@page "/page-with-script"
@using BlazorPageScript

<PageTitle>Enhanced Load Script Example</PageTitle>

<PageScript Src="./Components/Pages/PageWithScript.razor.js" />

Welcome to my page.

In the Blazor Web App, add the following collocated JS file:

• onLoad is called when the script is added to the page.
• onUpdate is called when the script still exists on the page after an enhanced update.
• onDispose is called when the script is removed from the page after an enhanced update.

Components/Pages/PageWithScript.razor.js:

export function onLoad() {
  console.log('Loaded');
}

export function onUpdate() {
  console.log('Updated');
}

export function onDispose() {
  console.log('Disposed');
}

In a Razor Class Library (RCL) (the example RCL is named BlazorPageScript), add the following module.

wwwroot/BlazorPageScript.lib.module.js:

const pageScriptInfoBySrc = new Map();

function registerPageScriptElement(src) {
  if (!src) {
    throw new Error('Must provide a non-empty value for the "src" attribute.');
  }

  let pageScriptInfo = pageScriptInfoBySrc.get(src);

  if (pageScriptInfo) {
    pageScriptInfo.referenceCount++;
  } else {
    pageScriptInfo = { referenceCount: 1, module: null };
    pageScriptInfoBySrc.set(src, pageScriptInfo);
    initializePageScriptModule(src, pageScriptInfo);
  }
}

function unregisterPageScriptElement(src) {
  if (!src) {
    return;
  }

  const pageScriptInfo = pageScriptInfoBySrc.get(src);
  
  if (!pageScriptInfo) {
    return;
  }

  pageScriptInfo.referenceCount--;
}

async function initializePageScriptModule(src, pageScriptInfo) {
  if (src.startsWith("./")) {
    src = new URL(src.substr(2), document.baseURI).toString();
  }

  const module = await import(src);

  if (pageScriptInfo.referenceCount <= 0) {
    return;
  }

  pageScriptInfo.module = module;
  module.onLoad?.();
  module.onUpdate?.();
}

function onEnhancedLoad() {
  for (const [src, { module, referenceCount }] of pageScriptInfoBySrc) {
    if (referenceCount <= 0) {
      module?.onDispose?.();
      pageScriptInfoBySrc.delete(src);
    }
  }

  for (const { module } of pageScriptInfoBySrc.values()) {
    module?.onUpdate?.();
  }
}

export function afterWebStarted(blazor) {
  customElements.define('page-script', class extends HTMLElement {
    static observedAttributes = ['src'];

    attributeChangedCallback(name, oldValue, newValue) {
      if (name !== 'src') {
        return;
      }

      this.src = newValue;
      unregisterPageScriptElement(oldValue);
      registerPageScriptElement(newValue);
    }

    disconnectedCallback() {
      unregisterPageScriptElement(this.src);
    }
  });

  blazor.addEventListener('enhancedload', onEnhancedLoad);
}

In the RCL, add the following PageScript component.

PageScript.razor:

<page-script src="@Src"></page-script>

@code {
    [Parameter]
    [EditorRequired]
    public string Src { get; set; } = default!;
}

The PageScript component functions normally on the top-level of a page.

If you place the PageScript component in an app's layout (for example, MainLayout.razor), which results in a shared PageScript among pages that use the layout, then the component only runs onLoad after a full page reload and onUpdate when any enhanced page update occurs, including enhanced navigation.

To reuse the same module among pages, but have the onLoad and onDispose callbacks invoked on each page change, append a query string to the end of the script so that it's recognized as a different module. An app could adopt the convention of using the component's name as the query string value. In the following example, the query string is "counter" because this PageScript component reference is placed in a Counter component. This is merely a suggestion, and you can use whatever query string scheme that you prefer.

<PageScript Src="./Components/Pages/PageWithScript.razor.js?counter" />

To monitor changes in specific DOM elements, use the MutationObserver pattern in JS on the client. For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

Example implementation without using an RCL

The approach described in this article can be implemented directly in a Blazor Web App without using a Razor class library (RCL). For an example, see Enable QR code generation for TOTP authenticator apps in an ASP.NET Core Blazor Web App.