function connection_sql(text = 'select now();', values = []){
    row = {};
    const { Pool, Client } = require('pg');

    const client = new Client({
      user: 'cicada',
      host: '192.168.50.17',
      database: 'test',
      password: '123456',
      port: '5432'
    });
    
    //callback 
    client.connect();
    client.query(text, values, (err, res) => {     
       if (err) {
       console.log(err.stack);
     } else {
       row = res.rows;                
       console.log(row);       //выводит то что надо
     }
     client.end();     
    });
    return row;
}


module.exports.get_arr_db = function(){
    let arr = [];
    const text = 'SELECT temp1 FROM public.testdata limit 100;';
    temp = connection_sql(text);    
    arr = temp.temp1;
    console.log("arr = ", arr);  //здесь пустой массив
    return arr;
};


Console.log
arr =  undefined
[ { temp1: 420 },
  { temp1: 423 },
  { temp1: 425 },
  { temp1: 426 },
  { temp1: 428 },
  { temp1: 426 },
...