Promise chaining in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a resolved promise.

Node.js

const promise = Promise.[1]('Done');

Drag options to blanks, or click blank then click option'

Areject

Bresolve

Call

Drace

Attempts:

3 left

💡 Hint

Common Mistakes

Using reject instead of resolve

Using Promise.all or Promise.race incorrectly

The Promise.resolve() method creates a promise that is resolved with the given value.

2fill in blank

medium

Complete the code to add a handler for the resolved value.

Node.js

promise.[1](result => console.log(result));

Drag options to blanks, or click blank then click option'

Athen

Bcatch

Cfinally

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch instead of then

Using finally which runs regardless of outcome

The then method adds a callback for when the promise is fulfilled.

3fill in blank

hard

Fix the error in chaining a promise to handle rejection.

Node.js

promise.then(result => console.log(result)).[1](error => console.error(error));

Drag options to blanks, or click blank then click option'

Athen

Bfinally

Ccatch

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using then instead of catch for errors

Using finally which does not handle errors

The catch method handles promise rejections or errors.

4fill in blank

hard

Fill both blanks to chain two promises where the second depends on the first.

Node.js

fetchData().[1](data => processData(data)).[2](result => console.log(result));

Drag options to blanks, or click blank then click option'

Athen

Bcatch

Cfinally

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch or finally in place of then in the chain

Using resolve which is not a method on promises

Use then to chain promises and handle their results sequentially.

5fill in blank

hard

Fill all three blanks to create a promise chain with success, error, and cleanup handlers.

Node.js

getUser().[1](user => getProfile(user.id)).[2](error => console.error(error)).[3](() => console.log('Done'));

Drag options to blanks, or click blank then click option'

Athen

Bcatch

Cfinally

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up catch and finally

Using resolve which is not a chain method

The chain uses then for success, catch for errors, and finally for cleanup.

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