Promise chaining in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Promise chain?

Consider the following Node.js code using Promise chaining. What will be logged to the console?

Node.js

Promise.resolve(5)
  .then(x => x * 2)
  .then(x => { console.log(x); return x + 1; })
  .then(x => console.log(x));

A 11

B 5\n6

C 10

D 10\n11

💡 Hint

Remember each then returns a new Promise with the returned value.

The first then doubles 5 to 10. The second then logs 10 and returns 11. The third then logs 11.

📝 Syntax

intermediate

Which option causes a syntax error in Promise chaining?

Identify which code snippet will cause a syntax error when chaining Promises in Node.js.

A Promise.resolve(1).then(x => { return x + 1 }).then(x => console.log(x));

B Promise.resolve(1).then(x => x + 1).then(x => console.log(x))

C Promise.resolve(1).then(x => x + 1).then(x => { console.log(x) });

D Promise.resolve(1).then(x => x + 1).then(x => console.log(x));

💡 Hint

Look carefully at parentheses and braces.

Option B is missing a closing parenthesis at the end, causing a syntax error.

🔧 Debug

advanced

Why does this Promise chain not log the expected value?

Given the code below, why does the console.log output 'undefined' instead of the expected number?

Node.js

Promise.resolve(3)
  .then(x => { x * 3; })
  .then(x => console.log(x));

A The first then does not return a value, so x is undefined in the next then.

B Promise.resolve(3) is rejected, so then is skipped.

C console.log is called before the Promise resolves.

D The second then has a syntax error causing undefined output.

💡 Hint

Check if the first then returns a value explicitly.

Without a return statement, the first then returns undefined, so the next then receives undefined.

❓ state_output

advanced

What is the final value of 'result' after this Promise chain?

Analyze the code and determine the value of the variable 'result' after the Promise chain completes.

Node.js

let result = 0;
Promise.resolve(2)
  .then(x => x + 3)
  .then(x => { result = x * 2; })
  .then(() => console.log('Done'));

A 10

B 0

C NaN

D 5

💡 Hint

Follow the values through each then and how 'result' is assigned.

2 + 3 = 5, then result = 5 * 2 = 10.

🧠 Conceptual

expert

Which option correctly handles errors in a Promise chain?

Choose the code snippet that properly catches errors occurring in any step of the Promise chain.

A

Promise.resolve()
  .then(() => { throw new Error('Fail'); })
  .then(null, err => console.log('Caught:', err.message));

B

Promise.resolve()
  .then(() => { throw new Error('Fail'); })
  .catch(err => console.log('Caught:', err.message));

C

Promise.resolve()
  .then(() => { throw new Error('Fail'); })
  .then(() => console.log('No error'))
  .catch(err => console.log('Caught:', err.message));

D

Promise.resolve()
  .catch(err => console.log('Caught:', err.message))
  .then(() => { throw new Error('Fail'); });

💡 Hint

Errors thrown in any then should be caught by catch at the end.

Option C throws an error in a then, which is caught by the final catch. Option C also catches but does not continue chaining after error. Option C uses second then argument which only catches errors in previous Promise, not in the chain. Option C's catch is before the error throw, so it won't catch it.

Practice

1. What is the main purpose of promise chaining in Node.js?

easy

A. To convert synchronous code into asynchronous code

B. To run all asynchronous tasks at the same time

C. To run asynchronous tasks one after another in order

D. To stop all asynchronous tasks immediately

Solution

1. Step 1: Understand asynchronous task execution

   Promise chaining allows tasks to run one after another, waiting for each to finish.

2. Step 2: Identify the purpose of chaining

   Chaining with .then() ensures order, not parallel or immediate stop.

3. Final Answer:

   To run asynchronous tasks one after another in order -> Option C

4. Quick Check:

   Promise chaining = ordered async tasks [OK]

Hint: Promise chaining runs tasks step-by-step, not all at once [OK]

Common Mistakes:

• Thinking promises run in parallel by default
• Confusing chaining with synchronous loops
• Believing chaining stops tasks immediately

2. Which of the following is the correct syntax to chain two promises promise1 and promise2?

easy

A. promise1.then(promise2)

B. promise1.then(promise2())

C. promise1.then.then(promise2)

D. promise1.then(() => promise2())

Solution

1. Step 1: Understand how to pass functions to .then()

   The .then() method expects a function to call when the promise resolves.

2. Step 2: Check each option's syntax

   promise1.then(() => promise2()) correctly passes a function that calls promise2(). Options A and B call promise2() immediately or pass wrong types. promise1.then.then(promise2) has invalid chaining syntax.

3. Final Answer:

   promise1.then(() => promise2()) -> Option D

4. Quick Check:

   Pass a function to then() = promise1.then(() => promise2()) [OK]

Hint: Use a function inside then() to delay calling next promise [OK]

Common Mistakes:

• Calling the next promise immediately inside then()
• Using double then without parentheses
• Passing promise instead of function to then()

3. What will be logged to the console when running this code?

Promise.resolve(5)
  .then(x => x + 1)
  .then(x => { throw new Error('Fail'); })
  .catch(err => 'Caught: ' + err.message)
  .then(x => console.log(x));

medium

A. Caught: Fail

B. Fail

C. 6

D. undefined

Solution

1. Step 1: Follow the promise chain step-by-step

   The first then adds 1 to 5, resulting in 6. The second then throws an error.

2. Step 2: Understand error handling and final output

   The catch catches the error and returns the string 'Caught: Fail'. The last then logs this string.

3. Final Answer:

   Caught: Fail -> Option A

4. Quick Check:

   Error caught and message logged = Caught: Fail [OK]

Hint: Errors jump to catch, then continue chain with catch result [OK]

Common Mistakes:

• Expecting error to stop all logging
• Thinking catch returns undefined
• Ignoring that catch returns a value to next then

4. Identify the error in this promise chain:

fetchData()
  .then(data => processData(data))
  .then(result => console.log(result))
  .catch(console.error())

medium

A. Missing return in first then

B. Incorrect use of catch with immediate function call

C. processData is not a function

D. console.log should be inside catch

Solution

1. Step 1: Check the catch usage

   The catch method expects a function reference, but console.error() calls the function immediately, passing its result (undefined) instead.

2. Step 2: Correct catch usage

   It should be .catch(console.error) without parentheses to pass the function itself.

3. Final Answer:

   Incorrect use of catch with immediate function call -> Option B

4. Quick Check:

   Pass function to catch, not call it immediately [OK]

Hint: Pass function to catch, don't call it with () [OK]

Common Mistakes:

• Calling catch handler immediately instead of passing function
• Forgetting to return promises inside then
• Misplacing console.log inside catch

5. Given these functions:

function step1() { return Promise.resolve(2); }
function step2(x) { return Promise.resolve(x * 3); }
function step3(x) { return x + 4; }

What will this code log?

step1()
  .then(x => step2(x))
  .then(x => step3(x))
  .then(console.log);

hard

A. 10

B. Promise { 10 }

C. undefined

D. Error: step3 must return a Promise

Solution

1. Step 1: Calculate step1 and step2 results

   step1() resolves to 2. Then step2(2) resolves to 6 (2*3).

2. Step 2: Understand step3 return and final log

   step3(6) returns 10 (6+4) synchronously. Since then accepts a value or promise, it passes 10 to next then which logs 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Sync return in then passes value = 10 [OK]

Hint: then can handle sync values; they become resolved promises [OK]

Common Mistakes:

• Expecting step3 to return a Promise always
• Thinking console.log logs a Promise object
• Confusing synchronous return with Promise return