Promise chaining in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a handler that logs the resolved value of the promise.

Javascript

Promise.resolve('Hello').[1](value => console.log(value));

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

Athen

Bcatch

Cfinally

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch instead of then to handle success.

Using finally which does not receive the resolved value.

Trying to call resolve on the promise instance.

The then method is used to handle the resolved value of a promise.

2fill in blank

medium

Complete the code to chain a second then that doubles the number.

Javascript

Promise.resolve(5).then(num => num).[1](num => num * 2);

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

Aresolve

Bfinally

Ccatch

Dthen

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch instead of then for chaining.

Using finally which does not modify the value.

Trying to call resolve which is not a method on the promise instance.

To chain operations on promises, use then again.

3fill in blank

hard

Fix the error in the code by completing the blank with the correct method to catch errors.

Javascript

Promise.reject('Error occurred').[1](error => console.log(error));

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

Athen

Bfinally

Ccatch

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using then which only handles success.

Using finally which does not receive error arguments.

Trying to call resolve which is not a method on the promise instance.

The catch method handles rejected promises (errors).

4fill in blank

hard

Fill both blanks to create a promise chain that adds 3 then multiplies by 2.

Javascript

Promise.resolve(4).[1](num => num + 3).[2](num => num * 2);

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

Athen

Bcatch

Cfinally

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch in the middle of the chain which breaks the flow.

Using finally which does not transform the value.

Trying to call resolve which is not a method on the promise instance.

Both steps use then to process the resolved value sequentially.

5fill in blank

hard

Fill all three blanks to chain promises that fetch data, parse JSON, and log the result.

Javascript

fetch('url').[1](response => response.[2]()).[3](data => console.log(data));

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

Athen

Bjson

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using catch instead of then for success handlers.

Trying to call json as a property instead of a method.

Missing parentheses after json.

Use then to handle promises, json() to parse response, and another then to log data.

Practice

1. What does promise chaining allow you to do in JavaScript?

easy

A. Run multiple synchronous tasks at the same time

B. Stop all asynchronous tasks immediately

C. Run asynchronous tasks one after another, passing results along

D. Convert promises into synchronous functions

Solution

1. Step 1: Understand the purpose of promise chaining

   Promise chaining is designed to run async tasks in sequence, where each step waits for the previous to finish and can use its result.

2. Step 2: Compare options with the concept

   Only Run asynchronous tasks one after another, passing results along correctly describes running async tasks one after another and passing results along.

3. Final Answer:

   Run asynchronous tasks one after another, passing results along -> Option C

4. Quick Check:

   Promise chaining = sequential async tasks [OK]

Hint: Promise chaining runs async tasks in order [OK]

Common Mistakes:

• Thinking promises run synchronously
• Believing chaining stops tasks immediately
• Confusing promise chaining with parallel execution

2. Which of the following is the correct syntax to chain two promises p1 and p2?

easy

A. p1.then().then(p2).then(result => console.log(result));

B. p1.then(() => p2).then(result => console.log(result));

C. p1.then(p2).then(result => console.log(result));

D. p1.then(result => p2).catch(console.log);

Solution

1. Step 1: Understand promise chaining syntax

   To chain promises, the first promise's then should return the second promise function call, so it waits for it.

2. Step 2: Analyze each option

   p1.then(() => p2).then(result => console.log(result)); correctly returns p2 inside then. p1.then(p2).then(result => console.log(result)); passes p2 directly, which is incorrect. p1.then().then(p2).then(result => console.log(result)); has an empty then() which is invalid. p1.then(result => p2).catch(console.log); uses catch prematurely and returns p2 incorrectly.

3. Final Answer:

   p1.then(() => p2).then(result => console.log(result)); -> Option B

4. Quick Check:

   Return next promise in then = correct chaining [OK]

Hint: Return next promise inside then() for chaining [OK]

Common Mistakes:

• Passing promise directly to then instead of a function
• Leaving then() empty
• Misplacing catch before chain ends

3. What will be the output of this code?

Promise.resolve(5)
  .then(x => x * 2)
  .then(x => Promise.resolve(x + 3))
  .then(console.log);

medium

A. Error

B. 10

C. Promise { 13 }

D. 13

Solution

1. Step 1: Trace each then step

   First, Promise.resolve(5) resolves with 5. Then x => x * 2 returns 10. Next, x => Promise.resolve(x + 3) returns a promise resolving to 13.

2. Step 2: Understand final then output

   The last then(console.log) logs the resolved value 13, not a promise object.

3. Final Answer:

   13 -> Option D

4. Quick Check:

   Promise chaining unwraps nested promises [OK]

Hint: Promises unwrap nested promises automatically [OK]

Common Mistakes:

• Thinking console.log prints a Promise object
• Ignoring that then unwraps promises
• Confusing multiplication and addition order

4. Find the error in this promise chain:

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

function processData(data) {
  throw new Error('Processing failed');
}

medium

A. Error is caught by catch because thrown inside then chain

B. catch should be before then to catch errors

C. Missing return statement in processData causes error

D. Error is not caught because processData is synchronous

Solution

1. Step 1: Understand error throwing in promise chain

   Throwing an error inside processData causes the promise to reject, which is caught by the catch at the end.

2. Step 2: Analyze catch placement and behavior

   The catch at the end catches any error thrown in previous then callbacks, including synchronous throws.

3. Final Answer:

   Error is caught by catch because thrown inside then chain -> Option A

4. Quick Check:

   Thrown errors in then are caught by catch [OK]

Hint: Errors thrown in then are caught by catch at chain end [OK]

Common Mistakes:

• Thinking synchronous errors are not caught
• Placing catch before then incorrectly
• Assuming missing return causes error

5. You want to run three async tasks task1, task2, and task3 in order, where each depends on the previous result. Which code correctly chains them and catches any error once?

hard

A. task1()
.then(result1 => task2(result1))
.then(result2 => task3(result2))
.catch(error => console.error(error));

B. task1()
.then(() => task2())
.then(() => task3())
.catch(error => console.error(error));

C. task1()
.then(result1 => task2(result1))
.catch(error => console.error(error))
.then(result2 => task3(result2));

D. task1()
.catch(error => console.error(error))
.then(result1 => task2(result1))
.then(result2 => task3(result2));

Solution

1. Step 1: Understand dependency and error handling

   Each task depends on the previous result, so each then must pass the result to the next task function. A single catch at the end handles all errors.

2. Step 2: Analyze each option

   task1() .then(result1 => task2(result1)) .then(result2 => task3(result2)) .catch(error => console.error(error)); correctly chains tasks with result passing and one catch at the end. task1() .then(() => task2()) .then(() => task3()) .catch(error => console.error(error)); passes task2 and task3 without arguments, which may fail if they expect parameters. task1() .then(result1 => task2(result1)) .catch(error => console.error(error)) .then(result2 => task3(result2)); places catch too early, breaking the chain. task1() .catch(error => console.error(error)) .then(result1 => task2(result1)) .then(result2 => task3(result2)); places catch before then, so errors in later tasks are not caught.

3. Final Answer:

   task1().then(result1 => task2(result1)).then(result2 => task3(result2)).catch(error => console.error(error)); -> Option A

4. Quick Check:

   Chain with single catch at end and pass results [OK]

Hint: Chain with one catch at end, pass results explicitly [OK]

Common Mistakes:

• Placing catch too early breaking chain
• Not passing results to dependent tasks
• Multiple catches causing partial error handling