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TypeScript / Implement Rust-style Result

Posted On 02.14.2022

In Rust, there is a Result<T, E>◹ type that could return either an Ok(T) value if the operation is succeeded, or an Err(E) value if the operation is failed.

To mimic this type in TypeScript, first, let’s define the Result<T, E> type:

type Result<T, E = undefined> = { ok: true, value: T }
                            | { ok: false, error: E | undefined };

OK, so what’s going on here?

The Result<T, E> type is a union type that either returns:

  • A success result: which is an object that has the field ok set to true, and the field value of type T to carry the actual result.
  • The error result: which is an object that has the field ok set to false, and the field error of type E to carry the error details.

Following Rust’s Result type, we can omit the type E and write Result<T>. To support this, the error field needs to have the type of E | undefined. And defaults to undefined if not specified.

We also need a way to create a success or error value, let’s call them Ok(data: T) and Err(error?: E). Both functions return a Result<T, E> type:

const Ok = <T>(data: T): Result<T, never> => {
    return { ok: true, value: data };
};
 
const Err = <E>(error?: E): Result<never, E> => {
    return { ok: false, error };
};

For the Ok function, the error type E will never be used, so we don’t need to worry about the E type, hence the return type Result<T, never>. The same applied for the Result<never, E> type of the Err function.

Now, we can use our Result type, for example, let’s create a parseBoolean method that will return either a boolean value or an error string:

const parseBoolean = (input: string): Result<boolean, string> => {
    if ((/(true|false)/i).test(input)) {
        return Ok(input.toLowerCase() === "true");
    }
    return Err("Invalid input!");
}
 
console.log(parseBoolean("true"));
// -> Result { ok: true, value: true }
 
console.log(parseBoolean("False"));
// -> Result { ok: true, value: false }
 
console.log(parseBoolean("what"));
// -> Result { ok: false, error: 'Invalid input!' }
 
console.log(parseBoolean("haha"));
// -> Result { ok: false, error: 'Invalid input!' }

Or another example, this time we return a custom error type:

enum IntegerParseError {
    InvalidInput,
    NegativeNumber
};
 
const parseUnsignedInteger = (input: string): Result<number, IntegerParseError> => {
    let num = parseInt(input);
    if (isNaN(num)) {
        return Err(IntegerParseError.InvalidInput);
    }
    if (num < 0) {
        return Err(IntegerParseError.NegativeNumber);
    }
    return Ok(num);
}
 
console.log(parseUnsignedInteger("hello"));
// -> Result { ok: false, error: IntegerParseError.InvalidInput }
 
console.log(parseUnsignedInteger("1238"));
// -> Result { ok: true, value: 1238 }
 
console.log(parseUnsignedInteger("-12"));
// -> Result { ok: false, error: IntegerParseError.NegativeNumber }

Oh, and we can omit the Err type as well, it will be defaulted as undefined:

const parseBooleanTheYoloWay = (input: string): Result<boolean> => {
    if ((/(true|false)/i).test(input)) {
        return Ok(input.toLowerCase() === "true");
    }
    return Err();
}
 
console.log(parseBooleanTheYoloWay("haha"));
// -> Result { ok: false, error: undefined }

And that’s it, we have implemented Rust’s Result type in TypeScript, with just 7 lines of code! (Well… just the spirit of it, there are a lot more about the real Result type of course).

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