How do I fix the error "Argument of type 'MonoTypeOperatorFunction<RouterEvent>' is not assignable to parameter of type 'OperatorFunction<Event_2, RouterEvent>'" in Angular?

Question

How do I fix the error "Argument of type 'MonoTypeOperatorFunction<RouterEvent>' is not assignable to parameter of type 'OperatorFunction<Event_2, RouterEvent>'" in Angular?

2 Answers

2 answers

How do I fix the error "Argument of type 'MonoTypeOperatorFunction<RouterEvent>' is not assignable to parameter of type 'OperatorFunction<Event_2, RouterEvent>'" in Angular?

0

The error "Argument of type 'MonoTypeOperatorFunction' is not assignable to parameter of type 'OperatorFunction'" in Angular typically arises when there's a mismatch in the expected and provided types when using RxJS operators, particularly within Angular's routing context. Here's a breakdown of the causes and solutions:

Likely Cause

RxJS Version Mismatch: Angular relies heavily on RxJS. A discrepancy between the Angular version and the RxJS version it's using can lead to type compatibility issues. Older versions of RxJS had different type definitions for operators.
Incorrect Operator Usage: You might be using an operator that transforms the stream's type in a way that doesn't align with what the subsequent operator expects. For instance, if you have a stream of `RouterEvent` and an operator inadvertently changes it to a more generic `Event`, you'll see this error.
Type Inference Problems: Sometimes, TypeScript's type inference struggles to correctly determine the types flowing through your observable chain, especially with complex transformations.

Solutions

Update RxJS: This is the most common fix. Ensure your RxJS version is compatible with your Angular version.

To update RxJS:
npm install rxjs@latest
Important: Check Angular's compatibility matrix for the recommended RxJS version for your specific Angular version.
Explicit Type Assertions: Use type assertions to explicitly tell TypeScript the type of the observable stream at a particular point. This helps resolve type inference issues.

Example:
this.router.events.pipe( filter(e => e instanceof NavigationEnd) as OperatorFunction, // ... other operators )
In this example, we're asserting that the output of the `filter` operator is a stream of `NavigationEnd` events.
Type Guards: If you are using `filter`, use a type guard to narrow down the type.

Example:
function isNavigationEnd(event: RouterEvent): event is NavigationEnd { return event instanceof NavigationEnd; } this.router.events.pipe( filter(isNavigationEnd), // ... other operators. TypeScript now knows it's a NavigationEnd );
Review Operator Logic: Carefully examine the operators in your `pipe`. Make sure each operator is handling the stream's type correctly and that the output type of one operator aligns with the expected input type of the next. If an operator is unintentionally changing the type, you might need to adjust its logic or introduce a mapping operator (`map`) to restore the expected type.
Minimal Reproduction: If the problem persists, try to isolate the issue by creating a minimal, reproducible example. This helps pinpoint the exact location of the type mismatch. You can use StackBlitz or a similar online editor to share the example.

Example Scenario and Solution

Let's say you have code like this:


   this.router.events.pipe(
   filter(event => event instanceof NavigationEnd),
   // some other operator that expects RouterEvent
   ).subscribe(...);

The `filter` operator, without a type guard, might be seen as potentially returning a generic `Event` instead of a `RouterEvent`. The solution is to use a type assertion or a type guard as shown above.

Debugging Tips

Console Logging: Add `tap` operators at various points in your observable chain to log the type of the emitted values. This can help you see where the type is changing unexpectedly.
TypeScript Compiler Options: Ensure your `tsconfig.json` has strict type checking enabled (`"strict": true`). This will help TypeScript catch more type-related errors.

By systematically checking these areas, you should be able to resolve the "Argument of type 'MonoTypeOperatorFunction' is not assignable to parameter of type 'OperatorFunction'" error.

Additional Resources

Wrote answer · 3/14/2025

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Related Questions

चंद्रशेखर गारकर · Accepted Answer · Wrote answer · 5/27/2023

After updating angular to the latest version, you might get this error:

Argument of type 'MonoTypeOperatorFunction<RouterEvent>' is not assignable to parameter of type 'OperatorFunction<Event_2, RouterEvent>'.

Types of parameters 'source' and 'source' are incompatible.

Type 'Observable<Event_2>' is not assignable to type 'Observable<RouterEvent>'.

Type 'Event_2' is not assignable to type 'RouterEvent'.

Type 'RouteConfigLoadStart' is missing the following properties from type 'RouterEvent': id, url

This error occurs at line where to subscribe to router events, that looks something like this:

this.routeSub = this.router.events

.pipe(

filter(

(event: RouterEvent) => event instanceof NavigationStart

),

tap(() => this.trigger.closePanel())

)

.subscribe();

Solution 1:

In recent version of angular, router.events emit a generic Event and not RouterEvent. Hence to correctly fix the issue import the Event from the angular/router module and replace RouterEvent with Event.

Here is how you do it:

import { NavigationStart, Router, Event} from "@angular/router";

this.routeSub = this.router.events

.pipe(

filter(

(event: Event) => event instanceof NavigationStart

),

tap(() => this.trigger.closePanel())

)

.subscribe();

Solution 2:

If you really don't care about explicit type(e.g. in the filter function above), just remove the RouterEvent type, and compiler will implicitly assign the Event type to it.