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.

Practice

1. What is the main purpose of the pg library in Node.js?

easy

A. To connect and interact with PostgreSQL databases

B. To create web servers

C. To manage file uploads

D. To style web pages

Solution

1. Step 1: Understand the role of pg

   The pg library is designed specifically to connect Node.js applications to PostgreSQL databases.

2. Step 2: Compare with other options

   Creating web servers, managing file uploads, and styling pages are unrelated to database connections and are handled by other libraries.

3. Final Answer:

   To connect and interact with PostgreSQL databases -> Option A

4. Quick Check:

   pg = PostgreSQL connection [OK]

Hint: Remember pg is for PostgreSQL database connection [OK]

Common Mistakes:

• Confusing pg with web server libraries
• Thinking pg handles frontend styling
• Assuming pg manages file uploads

2. Which of the following is the correct way to create a new Client instance from the pg library?

easy

A. const client = pg.connect('postgresql://user:pass@localhost/db');

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

C. const client = new Client.connect('postgresql://user:pass@localhost/db');

D. const client = pg.new Client('postgresql://user:pass@localhost/db');

Solution

1. Step 1: Recall the correct Client creation syntax

   The pg library requires creating a new Client instance using new pg.Client() with a config object.

2. Step 2: Check each option's syntax

   const client = new pg.Client({ connectionString: 'postgresql://user:pass@localhost/db' }); correctly uses new pg.Client({ connectionString: ... }). Options B, C, and D use invalid syntax or methods that do not exist.

3. Final Answer:

   const client = new pg.Client({ connectionString: 'postgresql://user:pass@localhost/db' }); -> Option B

4. Quick Check:

   Use new pg.Client with config object [OK]

Hint: Use 'new pg.Client' with config object for connection [OK]

Common Mistakes:

• Calling pg.connect directly without new Client
• Using incorrect method names like Client.connect
• Missing the 'new' keyword when creating Client

3. What will the following code output if the query succeeds?

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

async function run() {
  await client.connect();
  const res = await client.query('SELECT 1 AS number');
  console.log(res.rows[0].number);
  await client.end();
}

run();

medium

A. 1

B. [{ number: 1 }]

C. undefined

D. Error: query failed

Solution

1. Step 1: Understand the query and result structure

   The query SELECT 1 AS number returns one row with a column named 'number' having value 1.

2. Step 2: Check how the result is accessed

   The code logs res.rows[0].number, which accesses the first row's 'number' field, so it prints 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   res.rows[0].number = 1 [OK]

Hint: Access query result via res.rows[0].columnName [OK]

Common Mistakes:

• Logging res.rows instead of a specific field
• Expecting the whole array instead of one value
• Not awaiting client.connect() before query

4. Identify the error in this code snippet that uses pg to connect to PostgreSQL:

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

client.connect();
const res = await client.query('SELECT NOW()');
console.log(res.rows[0]);
client.end();

medium

A. client.end() should be called before query

B. client.query() should not be awaited

C. No error, code is correct

D. Missing await before client.connect()

Solution

1. Step 1: Check async usage for connect()

   The client.connect() method returns a promise and must be awaited to ensure connection before querying.

2. Step 2: Verify other async calls

   The await before client.query() is correct. Calling client.end() after query is also correct.

3. Final Answer:

   Missing await before client.connect() -> Option D

4. Quick Check:

   Always await client.connect() before queries [OK]

Hint: Always await client.connect() before querying [OK]

Common Mistakes:

• Not awaiting client.connect() causing race conditions
• Awaiting client.query() is correct, so skipping it is wrong
• Calling client.end() too early before query finishes

5. You want to write a Node.js function using pg that connects to PostgreSQL, runs a query, and always closes the connection even if an error occurs. Which pattern is best?

hard

A. Call connect, query, and end sequentially without error handling

B. Only call client.end() if query succeeds, skip on error

C. Use try-catch-finally: connect in try, query in try, close in finally

D. Use callbacks instead of async/await for better error handling

Solution

1. Step 1: Understand error-safe connection handling

   To ensure the connection closes even if an error happens, use a try-catch-finally block where client.end() is called in the finally block.

2. Step 2: Evaluate options for best practice

   Use try-catch-finally: connect in try, query in try, close in finally correctly uses try-catch-finally. Call connect, query, and end sequentially without error handling lacks error handling. Only call client.end() if query succeeds, skip on error risks leaving connection open on errors. Use callbacks instead of async/await for better error handling is outdated compared to async/await.

3. Final Answer:

   Use try-catch-finally: connect in try, query in try, close in finally -> Option C

4. Quick Check:

   try-catch-finally ensures connection closes [OK]

Hint: Use try-catch-finally to always close connection [OK]

Common Mistakes:

• Not closing connection on errors
• Ignoring async/await for error handling
• Closing connection only on success