'.if()' Does Not Alert Typescript of Checks Being Made

[alrightsure]

Given the following interfaces:

interface PersonTable {
    id: Generated<number>;
    first_name: string;
    last_name: string;
    gender: "male" | "female" | "other";
}

interface Database {
    person: PersonTable;
}

const db = new Kysely<Database>({
    dialect: new PostgresDialect({
        pool: new Pool({
            host: ""
        })
    })
});

The following function should allow the variable 'firstName' to be used in the where clause

async function selectPerson(gender: "male" | "female" | "other", firstName?: string | null) {
    return await db
        .selectFrom("person")
        .where("gender", "=", gender)
        .if(firstName !== null && firstName !== undefined, qb => qb.where("first_name", "=", firstName))
        .execute();
}

However, the built in '.if()' function kysely does not alert typescript that 'firstName' is a string rather than null or undefined.

Argument of type 'string | null | undefined' is not assignable to parameter of type 'FilterValueExpressionOrList<From<Database, "person">, "person", "first_name">'.
  Type 'undefined' is not assignable to type 'FilterValueExpressionOrList<From<Database, "person">, "person", "first_name">'.ts(2345)

After playing with kysely for a bit, there doesn't seem to be a good way to do this other than have the entire query written twice with an external if check. Please let me know if I am misguided or doing something wrong.

Would love to use this library on a larger sized project at work, but there are a lot of instances where we need to check for optional props being passed to the calling function.

[alrightsure]

Another note, I have also tried the following:

async function selectPerson(gender: "male" | "female" | "other", firstName?: string | null) {
    const query = db.selectFrom("person").where("gender", "=", gender);

    if (firstName) {
        query.where("first_name", "=", firstName);
    }

    return await query.execute();
}

selectPerson("male", "charles");

However, the second where clause is completely ignored from the query, even when firstName is passed. It seems any part of the query must be added to the initial chain.

[alrightsure]

updating as open as I closed the issue by mistake.

[koskimas]

Unfortunately it's impossible to alert typescript of the checks. This is a perfect place for !:

.if(firstName !== null && firstName !== undefined, qb => qb.where("first_name", "=", firstName!))

Everything is immutable in kysely, so you need to assign the results of operations back:

async function selectPerson(gender: "male" | "female" | "other", firstName?: string | null) {
    let query = db.selectFrom("person").where("gender", "=", gender);

    if (firstName) {
        query = query.where("first_name", "=", firstName);
    }

    return await query.execute();
}

selectPerson("male", "charles");