Then and catch methods in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the then method do in JavaScript promises?

The then method runs a function when a promise is successfully completed (resolved). It lets you handle the result of the promise.

beginner

What is the purpose of the catch method in promises?

The catch method runs a function when a promise fails (is rejected). It helps you handle errors or problems.

intermediate

How do then and catch work together in promise chains?

You use then to handle success and catch to handle errors. This way, you can manage both outcomes in a clear, step-by-step way.

beginner

What happens if you don’t use catch with a promise?

If you don’t use catch, errors in the promise might go unnoticed, causing bugs or unhandled promise rejections.

beginner

Show a simple example of using then and catch with a promise.

Example:<br>

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error:', error));

What does the then method do in a promise?

A Handles the successful result of the promise

B Handles errors in the promise

C Stops the promise from running

D Creates a new promise

The then method runs when the promise is resolved successfully.

Which method is used to catch errors in a promise chain?

A finally

B then

C catch

D resolve

The catch method handles rejected promises or errors.

What will happen if a promise is rejected but there is no catch method?

A The promise will automatically retry

B The program will stop running

C The <code>then</code> method will handle the error

D The error will be ignored and may cause unhandled rejection warnings

Without catch, errors can go unnoticed and cause warnings.

Can you chain multiple then methods?

A No, chaining is only for <code>catch</code>

B Yes, to handle multiple steps after a promise resolves

C Yes, but only if you use <code>catch</code> first

D No, only one <code>then</code> is allowed

You can chain many then calls to process data step-by-step.

What does this code do?<br>

promise.then(fn1).catch(fn2);

A Runs fn1 if promise resolves, fn2 if promise rejects

B Runs fn2 if promise resolves, fn1 if promise rejects

C Runs both fn1 and fn2 regardless of promise result

D Runs neither fn1 nor fn2

then handles success with fn1, catch handles errors with fn2.

Explain how then and catch methods help manage asynchronous code in JavaScript.

Think about how you handle good and bad outcomes in real life.

Describe a simple example where you would use then and catch with a promise.

Imagine getting data from the internet and handling success or failure.