Typescript mutate object add property

Question

I have the following code:

type Data = {
    projectID: number;
    foo: string;
}

const projectID = 1;
const datas: Data[] = []
const addToProject = (a: Omit<Data, "projectID">) => {
    a.projectID = projectID; // Property 'projectID' does not exist on type 'Omit<Data, "projectID">'
    datas.push(a);
};

addToProject({
    foo: 'bar',
});

The idea is that the object passed to addToProject doesnt need a projectID property, since in the given context, the projectID is already known and can be added by the function itself.
Inside the function, a is converted from Omit<Data, "projectID"> to Data.

How can I tell Typescript that I am changing the type of a?

I could change the function body to:

datas.push({
    ...a,
    projectID
});

but for performance reasons I don't want to create a new object.

I could also change every a to a as Data

(a as Data).projectID = projectID;
datas.push(a as Data);

but my real function has a lot more occurrences of a and this doesn't seem right.

Answer 1

There you are

const addToProject = (a: Omit<Data, "projectID">) => {
  const r = a as Data;
  r.projectID = projectID; // Property 'projectID' does not exist on type 'Omit<Data, "projectID">'
  datas.push(r);
};

But, honestly, I'd just create a new object. The performance impact is minuscule and it outweighs the risk of mutating an argument.