Promise chaining in Javascript

Introduction

Promise chaining helps you run tasks one after another in order. It makes sure each step waits for the previous one to finish.

When you need to do several tasks that depend on each other, like loading data then processing it.

When you want to handle multiple asynchronous steps without deeply nested code.

When you want to catch errors from any step in a simple way.

When you want to keep your code clean and easy to read while working with promises.

Syntax

Javascript

const promise = new Promise((resolve, reject) => {
  // do something async
  resolve(value);
});

promise
  .then(result => {
    // handle result
    return nextValue;
  })
  .then(nextResult => {
    // handle next result
  })
  .catch(error => {
    // handle any error from above
  });

Each then returns a new promise, so you can chain many.

If a then returns a value, it becomes the input for the next then.

Examples

This example starts with a resolved promise with value 5, doubles it, then prints 10.

Javascript

Promise.resolve(5)
  .then(value => value * 2)
  .then(value => console.log(value));

This shows how errors in a then are caught by catch.

Javascript

Promise.resolve()
  .then(() => {
    throw new Error('Oops');
  })
  .catch(error => console.log(error.message));

Shows chaining with multiple then calls passing values along.

Javascript

Promise.resolve('start')
  .then(value => {
    console.log(value);
    return 'next';
  })
  .then(value => console.log(value));

Sample Program

This program gets a number asynchronously, then changes it step by step using promise chaining. Each step waits for the previous one.

Javascript

function fetchNumber() {
  return new Promise((resolve) => {
    setTimeout(() => resolve(10), 500);
  });
}

fetchNumber()
  .then(number => {
    console.log('First number:', number);
    return number * 3;
  })
  .then(newNumber => {
    console.log('After multiplying by 3:', newNumber);
    return newNumber - 5;
  })
  .then(finalNumber => {
    console.log('After subtracting 5:', finalNumber);
  })
  .catch(error => {
    console.error('Error:', error);
  });

Important Notes

Time complexity depends on the tasks inside each then, but chaining itself is fast.

Space complexity is low since promises handle state internally.

Common mistake: forgetting to return a value inside then causes the next then to get undefined.

Use promise chaining when you want clear, step-by-step async code instead of nested callbacks.

Summary

Promise chaining runs async tasks in order, passing results along.

Each then returns a new promise, allowing many steps.

Errors anywhere in the chain can be caught with a single catch.

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