Typescript returning array of objects with unknown keys-CodePudding

I'm new to TypeScript, and trying to slowly migrate an existing JS (react) project to TypeScript.

I have a function executing a query:

/**
 * Run a Query against the currently connected database
 * @param {String} sql Query to run
 * @param {Object[]} Array of objects to be used in the prepared statement
 * @returns {Promise<Object[]>} Promise resolving after query is done. Returns an array of arrays with the result.
 */
const query = (sql, params) => {
  //Database stuff
}

And I'm calling this in numerous locations:

  const result = await query('SELECT count(*) as cnt from Table id=?',[2]')
  return result[0].cnt

The last line gives an error, because the field 'cnt' is unknown. How to best solve this? The return type is always Object[], but the content ob the object depends on the query.

Any suggestions how to best move this code to TS?

CodePudding user response：

You should probably be telling you "Database stuff" what kind of return type it can expect:

const query = function <T>(sql:string, params:object):Array<T> {
  //Database stuff that probably has an overload that accepts `<T>`
}

// Or:
const query = <T,>(sql:string, params:object):Array<T> => {
  //Database stuff that probably has an overload that accepts `<T>`
}

Called like:

await query<MyDataRow>('SELECT ...');

CodePudding user response：

You have to use generics

const query = <T>(sql: string, params: Object[]): T[] => {
    return []
}

But function can return empty array. In this case you can check cnt property like this

const result = await query('SELECT count(*) as cnt from Table id=?',[2])
return result[0]?.cnt