According to a comment by martin on Stack Overflow, the issue with typescript rxjs version 6.6.0 subscription lies in using the incorrect type for complete. Instead of (value: unknown) => void, it should be () => void.
I would like to delve into why this error is displayed as it is.
The problematic method, subscribe, in RxJS has multiple signature overloads as found in its codebase here.
subscribe(observer?: PartialObserver<T>): Subscription;
/** @deprecated Use an observer instead of a complete callback */
subscribe(next: null | undefined, error: null | undefined, complete: () => void): Subscription;
/** @deprecated Use an observer instead of an error callback */
subscribe(next: null | undefined, error: (error: any) => void, complete?: () => void): Subscription;
/** @deprecated Use an observer instead of a complete callback */
subscribe(next: (value: T) => void, error: null | undefined, complete: () => void): Subscription;
subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription;
When calling this method, TypeScript evaluates the overloads sequentially from top to bottom.
The object passed by OP was
{
next: (value) => {},
error: (value) => {},
complete: (value) => {},
}
Initially, the inferred signatures were
{
next: (value: unknown) => void,
error: (value: unknown) => void,
complete: (value: unknown) => void,
}
Type inference occurs only when the correct overload is determined based on argument compatibility in the subscribe method.
As TypeScript matches the first signature, it expects a PartialObserver object which must include at least one of next, error, or complete properties. The provided object did not fit due to the wrong type for complete.
This mismatch could lead to silent errors in some TypeScript versions, causing confusion. Subsequent mismatches in other signatures may also occur depending on the version used.
In a similar example run on TS version 4.2.3, the error message is more detailed, showing clearer information compared to what the OP encountered with TS2554:
No overload matches this call.
Overload 1 of 5, '(observer?: PartialObserver<unknown> | undefined): Subscription', gave the following error.
Type '(value: any) => void' is not assignable to type '() => void'.
Overload 2 of 5, '(next?: ((value: unknown) => void) | undefined, error?: ((error: any) => void) | undefined, complete?: (() => void) | undefined): Subscription', gave the following error.
Argument of type '{ next: (value: unknown) => void; error: (value: any) => void; complete: (value: any) => void; }' is not assignable to parameter of type '(value: unknown) => void'.
Object literal may only specify known properties, and 'next' does not exist in type '(value: unknown) => void'.
To avoid such confusing errors, it's important to understand and evaluate all possible overloads while troubleshooting.