I have the following Typescript function that query a table in Postgresql;I pass a generic type and I expect to have an array of objects of the same type passed as input, but it doesn't:
export async function pgQuery<T>(
query: string,
postgreSqlClient: Client = postgreSqlClientGlobal
) {
const log = extendLogger(_log, pgQuery);
try {
const res = await postgreSqlClient.query<T>(query);
return res;
} catch (error) {
log.error({ error }, `PostgreSql query ERROR`);
throw new Error(`PostgreSql query ERROR - ${error}`);
}
}
The input type T is:
{country: string,
age_min: number,
age_max: number}
And in the table of the Postgresql DB I have records like this:
{
loc_name: "Albania",
age_min: 15,
age_max: 64
}
but I have, as output of the executed query, object like this:
{country: string,
age_min: string,
age_max: string}
Example:
{
loc_name: "Albania",
age_min: "15",
age_max: "64"
}
How to retrieve the correct types?
pg stands for: https://github.com/brianc/node-postgres
It depends on how postgreSqlClient.query<T>(query) is implemented.
When you interact with an external database, just typing the request as type T is not enough to guarantee that you will actually get a value of type T.
When you declare the type as T, Typescript will just trust you and assume that the type is T. However, since you are fetching an external resource, you cannot be sure at runtime that the value is actually of type T.
If you want to be 100% sure, you must implement some sort of runtime validation. Libraries like zod and io-ts are made for this purpose.
The fastest solution (I'm writing a "disposable code"...) is to cast the fields in the postgresql query (es.: select loc_name, age_min::int, age_max::int from table;); for more info see:
pg-promise returns integers as strings