How to force typescript to require to return undefined rather than having an explicit return?

Issue

Let's take this example:

function maybeString(a: number): string | undefined {
    if (a > 5) {
        return 'ok';
    }
}

This is valid Typescript.

Is there a configuration to force this code to be:

function maybeString(a: number): string | undefined {
    if (a > 5) {
        return 'ok';
    } else {
        return undefined;
    }
}

So always making the return undefined necessary and explicit?

Solution

Normally that should already lead to a TS error (link to TS playground). The error should be "Not all code paths return a value.ts(7030)".

Maybe check your tsconfig file and set "noImplicitReturns" to true. If not yet true, this should enable such warnings.

tsconfig.json:

{
  "compilerOptions": {
    "noImplicitReturns": true
  }
}

Answered By - ddemand