TypeScript generic constraints to multiple primitive types

Question

Given this function:

myFunc(object: string | null): string | null {}

I would like this function to be of return type string when the object is string, and return type of string | null when the object is of type string | null.

I have tried:

myFunc<T extends string | null>(object: T): T {
    return "returnValue"; //Type '"returnValue"' is not assignable to type 'T'.
}

and

myFunc<T extends string & null>(object: T): T {
    return "returnValue"; //Type '"returnValue"' is not assignable to type 'T'.
}

Both produce the same compile error. I have not found the correct syntax to do this.

Fenton · Accepted Answer · 2017-12-07 16:30:01Z

7

I can't believe I'm actually going to suggest overloads, as I spend a goodly portion of my time explaining how most of them can be avoided... but an overload would be a good way to model this!

class Example {
    myFunc(obj: string): string;
    myFunc(obj: string | null): string | null;
    myFunc(obj: string | null): string | null {
        return "returnValue";
    }
}

function test(a: string | null, b: string) {
    const example = new Example();

    // resultA: string | null
    const resultA = example.myFunc(a);

    // resultB: string
    const resultB = example.myFunc(b);
}

In the above example, the return types are mapped to the input types, so resultA and resultB have the expected types, provided you are running with strict null checks enabled.

answered Dec 7, 2017 at 16:30

Fenton

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