PostgreSQL connection with pg in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the correct way to create a PostgreSQL client using pg

Which option correctly creates a new PostgreSQL client instance using the pg library in Node.js?

A const pg = require('pg'); const client = new pg.Client({ connectionString: process.env.DATABASE_URL });

B import { Client } from 'pg'; const client = new Client(process.env.DATABASE_URL);

C const { Client } = require('pg'); const client = new Client({ connectionString: process.env.DATABASE_URL });

D import pg from 'pg'; const client = new pg.Client({ connectionString: process.env.DATABASE_URL });

💡 Hint

Remember that in Node.js with CommonJS, you use require and access Client from the pg module.

Options A and C correctly create a new Client instance. Option A imports the entire pg module and accesses Client via pg.Client. Option C destructures Client from require('pg') and uses it correctly. Option B uses ES module import but passes connection string directly without an object. Option D uses ES module import default which is not the standard way for pg.

❓ component_behavior

intermediate

What happens after calling client.connect()?

After creating a PostgreSQL client with pg and calling client.connect(), what is the expected behavior?

Node.js

const { Client } = require('pg');
const client = new Client({ connectionString: 'postgresql://user:pass@localhost/db' });
client.connect();

A The client immediately connects to the database and is ready to run queries synchronously.

B The client starts an asynchronous connection process; queries can be run after the connection is established.

C The client throws an error if the connection string is invalid but does not connect otherwise.

D The client queues queries automatically until the connection is established.

💡 Hint

Think about how asynchronous operations work in Node.js.

Calling client.connect() starts an asynchronous connection to the database. You should wait for the connection to complete before running queries. It does not connect synchronously (A). It does not throw immediately on invalid connection strings but will error asynchronously (C). It does not queue queries automatically (D).

🔧 Debug

advanced

Why does this pg client connection code throw an error?

Consider this code snippet. Why does it throw an error when trying to connect?

import { Client } from 'pg';
const client = new Client('postgresql://user:pass@localhost/db');
await client.connect();

Node.js

import { Client } from 'pg';
const client = new Client('postgresql://user:pass@localhost/db');
await client.connect();

A The Client constructor expects an object with a connectionString property, not a string directly.

B The connection string format is incorrect and causes a parsing error.

C The await keyword cannot be used outside an async function.

D The import statement is invalid syntax in Node.js.

💡 Hint

Check the expected argument type for the Client constructor.

The Client constructor requires an object with a connectionString property, not a plain string. Passing a string directly causes an error. The import syntax is valid in ES modules (B). Await outside async function (C) is a separate issue but not the main cause here. The connection string format is correct (D).

❓ state_output

advanced

What is the value of result.rows after this query?

Given the following code, what will result.rows contain after the query completes?

const { Client } = require('pg');
const client = new Client({ connectionString: process.env.DATABASE_URL });
await client.connect();
const result = await client.query('SELECT id, name FROM users WHERE id = $1', [42]);
await client.end();

Node.js

const { Client } = require('pg');
const client = new Client({ connectionString: process.env.DATABASE_URL });
await client.connect();
const result = await client.query('SELECT id, name FROM users WHERE id = $1', [42]);
await client.end();

A An array of all users ignoring the WHERE clause

B Undefined because query returns no rows property

C An empty array [] if no user with id 42 exists

D [{ id: 42, name: 'Alice' }]

💡 Hint

Think about what happens if the query finds no matching rows.

If no user with id 42 exists, result.rows will be an empty array. It will not be undefined (D). It will not contain all users (C). It contains matching rows only (A) if found.

🧠 Conceptual

expert

Which option correctly handles connection errors with pg client?

You want to connect to PostgreSQL using pg and handle connection errors gracefully. Which code snippet correctly does this using async/await?

A

try {
  await client.connect();
} catch (err) {
  console.error('Connection error', err);
  process.exit(1);
}

B

client.connect(err => {
  if (err) throw err;
  console.log('Connected');
});

C

await client.connect().catch(err => {
  console.error('Connection error', err);
  process.exit(1);
});

D client.connect().then(() => console.log('Connected')).catch(err => console.error(err));

💡 Hint

Focus on proper async/await error handling.

Option A uses try/catch with async/await to catch connection errors properly. Option A uses callback style, not async/await. Option A uses catch on await which is invalid syntax. Option A mixes promises without await and does not exit on error.