dotnet/aspnetcore · error · Error

Invalid JS call result type

Error message

Invalid JS call result type '${resultType}'.

What it means

Thrown by createJSCallResult() when the resultType passed from .NET does not match any JSCallResultType enum case (Default=0, JSObjectReference=1, JSStreamReference=2, JSVoidResult=3). This is a defensive default branch; the value is supplied by the .NET runtime when invoking JS, so an unknown value means a version/protocol mismatch between the JS library and the .NET runtime.

Solutions

1. Hard-refresh / clear cached Microsoft.JSInterop.js assets so the version matches the runtime.
2. Ensure the npm @microsoft/dotnet-js-interop package version aligns with the SDK version.
3. If hosting a custom dispatcher, only emit result type codes 0-3.

Example fix

// before (stale cached asset)
// browser serves old Microsoft.JSInterop.js with new runtime -> unknown resultType

// after
// clear cache / cache-bust the JS asset so it matches the runtime version
// e.g. append a version hash to the script src in index.html

// Ensure the served JS interop asset matches the runtime version.
// <script src="_framework/blazor.webassembly.js?v={CACHE_BUSTER}"></script>
// and verify window.DotNet.JSCallResultType has the expected enum members.

function isKnownResultType(t) { return t === 0 || t === 1 || t === 2 || t === 3; }

try { /* interop invoke */ } catch (e) { if (/Invalid JS call result type/.test(e.message)) { console.error('JS interop asset/runtime version mismatch. Clear cached assets and reload.'); } throw e; }

When it happens

Trigger: .NET invokes a JS function with a result type code outside the known enum range (e.g. 4 or higher from a newer runtime loaded against an older JS asset). The switch in createJSCallResult falls through to default and throws.

Common situations: JS interop static asset cached (stale) in the browser while the .NET runtime was upgraded to a version using a new result type; mismatched npm package version; custom host emitting an invalid result type code.

Related errors

• Byte array index ' ' does not exist.
• Cannot create a JSObjectReference from the value
• Cannot create a JSStreamReference from the value
• Cannot create a JSStreamReference from the value
• Invalid headers.

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/8bef133e27e56f59. Report an issue: GitHub.

Appendix: source

Thrown at src/JSInterop/Microsoft.JSInterop.JS/src/src/Microsoft.JSInterop.ts:805

          this.streamPromise = new Promise((resolve, reject) => {
              this.resolve = resolve;
              this.reject = reject;
          });
      }
  }

  function createJSCallResult(returnValue: any, resultType: JSCallResultType) {
      switch (resultType) {
      case JSCallResultType.Default:
          return returnValue;
      case JSCallResultType.JSObjectReference:
          return createJSObjectReference(returnValue);
      case JSCallResultType.JSStreamReference:
          return createJSStreamReference(returnValue);
      case JSCallResultType.JSVoidResult:
          return null;
      default:
          throw new Error(`Invalid JS call result type '${resultType}'.`);
      }
  }

  let nextByteArrayIndex = 0;
  function stringifyArgs(callDispatcher: CallDispatcher, args: any[] | null) {
      nextByteArrayIndex = 0;
      currentCallDispatcher = callDispatcher;
      const result = JSON.stringify(args, argReplacer);
      currentCallDispatcher = undefined;
      return result;
  }

  function getCaptureIdFromElement(element: Element): string | null {
      for (let i = 0; i < element.attributes.length; i++) {
          const attr = element.attributes[i];
          if (attr.name.startsWith("_bl_")) {
              return attr.name.substring(4);
          }

View on GitHub (pinned to 3600ca084e)