How do I get around this “Subject incorrectly extends Observable” error in TypeScript 2.4 and RxJS 5.x?

Question

Solution

RxJS 5.4.2 should now work perfectly with TypeScript 2.4.1. Simply upgrade to 5.4.2+ if possible.

npm install --save rxjs@^5.4.2

Then try restarting your editor and/or recompile if you don’t see an immediate change.

If not, the below solution should work.

TypeScript 2.4 has a strictness change, and Subject<T> isn’t lifting to the correct Observable. The signature really should have been

<R>(operator: Operator<T, R>) => Observable<R>

This will be fixed in RxJS 6.

Newer versions of RxJS will have this fixed, but as a temporary workaround, you can use the noStrictGenericChecks compiler option.

In tsconfig.json, put it in "compilerOptions" and set it to true.

{
    "compilerOptions": {
        "noStrictGenericChecks": true
    }
}

On the command line it’s --noStrictGenericChecks.