Typescript's contravariant object values

Here is an example of an overloaded Typescript function:

function clearField(o : Record<"field", string>, nullify : false) : void
function clearField(o : Record<"field", string | null>, nullify : true) : void
function clearField(o : any, nullify : any) : void
{
    if (nullify)
    {
        o.field = null;
    }
    else
    {
        o.field = "";
    }
}

Now, let's take a look at this object:

let o : Record<"field", string> = {"field" : "toBeCleared"};

Unfortunately, the following code will not produce any errors:

clearField(o, true); // 'o.field' is changed to null even though it was declared as type string.

To fix this issue and receive a Typescript error in strict mode, what changes can be made to the function declarations/definitions? The goal is to prevent the widening of Record<"field", string> to Record<"field", string | null> when passed to "clearField," maintaining contravariance for the values in that context.

typescriptjavascript-objectstypescript-genericscontravariance

Answer №1

To implement conditional logic based on the field value's nullability, you can utilize a constrained generic type parameter within your overload signature in TypeScript. This approach allows the compiler to determine whether the field value is nullable and handle it accordingly. By setting the parameter's type to 'never' in cases where the field value does not allow for null, you can effectively control how the function behaves:

Explore this concept further on the function eraseField(o: Record<"field", string>, nullify: false): void; function eraseField<T extends Record<"field", string | null>>( o: T extends Record<"field", string> ? never : T, nullify: true, ): void; function eraseField(o: Record<"field", string | null>, nullify: boolean) { if (nullify) o.field = null; else o.field = ""; }

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