Keep type information after creating spread copy

June 28, 2023

I have the problem, that I have a function that gets an object. A field of this object can be null, which I want to handle, and afterwards add a field to this object and return it.

Here you can find a simplified version of it.

The problem that I have is that the assertion that the field is not null is lost after I create the spread clone.

Is there a way to get around this?

>Solution :

The bar.field is string[] information is kept only in bar.field, not in bar itself

type Type1 = {
    field: string[] | null;
}

type Type2 = {
    name: string;
    field: string[]
}

function foo(bar: Type1): Type2 {
    bar.field ??= [];
    const result = { name: "some Name", ...bar, field: bar.field };
    result.field.push("Some String");
    return result;
}