Promise chaining in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is promise chaining in JavaScript?

Promise chaining is a way to run multiple asynchronous tasks one after another, where each task starts when the previous one finishes, using .then() methods.

beginner

How do you return a value from one promise to the next in a chain?

You return the value inside the .then() callback. This value becomes the input for the next .then() in the chain.

intermediate

What happens if a promise in the chain is rejected?

If a promise is rejected, the chain skips to the nearest .catch() handler to handle the error.

beginner

Why is promise chaining better than nested callbacks?

Promise chaining avoids deeply nested code (callback hell), making code easier to read and maintain by keeping asynchronous steps in a flat, linear structure.

beginner

Show a simple example of promise chaining with two asynchronous steps.

Example:

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

This runs fetchData, then processData with the fetched data, then logs the result, handling errors if any.

What method do you use to add the next step in a promise chain?

A .then()

B .catch()

C .finally()

D .start()

The .then() method adds the next step to a promise chain.

If a promise in the chain fails, which method handles the error?

A .then()

B .finally()

C .error()

D .catch()

The .catch() method handles errors in promise chains.

What does returning a promise inside a .then() do?

A Ends the chain

B Waits for that promise before continuing the chain

C Starts a new unrelated chain

D Ignores the returned promise

Returning a promise inside .then() makes the chain wait for it to resolve before moving on.

Which problem does promise chaining help to solve?

A Callback hell

B Memory leaks

C Synchronous blocking

D Variable hoisting

Promise chaining helps avoid deeply nested callbacks known as callback hell.

What will this code output?

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

A 5

B 10

C 11

D Error

The chain multiplies 5 by 2 (10), then adds 1 (11), then logs 11.

Explain how promise chaining works and why it is useful.

Think about how you do tasks one after another and how promises help with that.

Write a simple promise chain that fetches data, processes it, and logs the result, including error handling.

Start with a promise, then add steps with .then(), and end with .catch()

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