Return statement required even when not needed

[eliashakuni]

🔎 Search Terms

"Function lacks ending return statement and return type does not include 'undefined'."
"Not all code paths return a value."

🕗 Version & Regression Information

I have searched the FAQ and issues on GitHub, but couldn't find this question. I have seen similar things, but the arguments for the behavior in those doesn't apply in this case. I tried tried the latest and oldest versions of TS in the playground, and the issue was always there.

In other similar issues, there always seems to be a way to send in more than expected. For example, in #13215 it's about a generic function that can receive something else because of the "extends" keyword.

In #36884 it's because "instanceof" is open ended so to speak.

In my case, I'm talking about a closed set of primitives. In case it's just that I haven't been able to find the duplicates of this one, I'm sorry.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/ATAuE8AcFNgQQE4HMAqVYF5gAMBmB7fbYAHxwCMBDBbAbgCh6QBjfAOwGdQxouAxAK5tmoAJbtgWABTUkALnjI0MAJQKuCUWySSAfMADeTEMFG5gM5JIxY8hbCsPGTwBNFACEbHIh2iOwAREDC4AvowuZhay1rZUNI5GLiBuHl4+Vv7A8XTOwOF5APSFwAAS0G4ANMAxWfjmEDDAbNAAbhXGoQzGrJzczJQANoMVgsJiEtK4oghcCr7KsGTYVABe2NUc0L0AJuqgmtrVoAAWM3s1bOCOGPpJJqC8oGMi4mxS07OgKrTAxcAAUQQCHwCAAhEUSr4BABbaBsbj1MDoYAAcgWKLIACI1ljUaYAmx8NxKBwOKIkGxKOQRmB8MBINRKHDHghgEjGrB0Up0KiAHSQ4CLNE4yirPEE5rEmpkilUmmwUD0zlojEwfl5PKPfhCV7sKRbXY-P4lIEg8Fap4vCbvIQ7aDTFo7Y3-M2giEubXPXU2qQu03A90RB5Wn1vKSnc6Of4AeTYg3A7JaYBOlG4AHdQQBrDjVcgCbineHAcD4ATAVPtbKk0QDYaJgBWAi4YE8TvZuAaKOYJ22Wa0SE2yrOAQ4JzLgx2Uv6ZYRMpqwFYQlZwGgAA9mZARgKQF1GPQgA

💻 Code

  type ArgType = `foo` | `bar`;

  const testFunction = (arg: ArgType): string => {
    if (arg === `foo`) {
      return `Arg is foo`;
    }

    if (arg === `bar`) {
      return `Arg is bar`;
    }

    // Here, arg is of type never
  };

  const callerFunction = (first: ArgType | `baz`, second: string, third: any) => {
    testFunction(first); // Error!
    // Argument of type 'ArgType | "baz"' is not assignable to parameter of type 'ArgType'.
    // Type '"baz"' is not assignable to type 'ArgType'.
    
    testFunction(second); // Error!
    testFunction(undefined); // Error!
    testFunction(); // Error!

    testFunction(third) // Only one that works, but then you have basically just turned off type checking, so this 
    // should not count as a counter example.
  };

🙁 Actual behavior

The testFunction doesn't compile, returns "Function lacks ending return statement and return type does not include 'undefined'." error.

🙂 Expected behavior

testFunction should compile without errors.

Additional information about the issue

I run into this quite often in Angular with computed signals. If I add an ending return statement, it's wrong for several reasons.

1. It implies the function can return whatever value I would put in the end, which is not true.
2. Overall, it bloats the code, and unreachable code is not exactly best practice.
3. Most critically, the reason this is a more serious problem. If ArgType is expanded to 'foo' | 'bar' | 'baz', then there SHOULD be an error. If I have a return statement in the end, that's just hiding the error and I could just as well use // @ts-ignore.

[MartinJohns]

Duplicate of #21985.