Promise catch for async errors in Node.js - Step-by-Step Execution

Concept Flow - Promise catch for async errors

Start async operation

↓

Promise created

↓

Async operation succeeds?

No→Error thrown

↓

Catch error

↓

Resolve promise

↓

Handle error

↓

Then block runs

↓

End

This flow shows how a Promise runs an async task, then either resolves successfully or catches errors to handle them.

Execution Sample

Node.js

function fetchData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => reject('Network error'), 1000);
  });
}

fetchData()
  .then(data => console.log('Data:', data))
  .catch(err => console.log('Error:', err));

This code creates a Promise that fails after 1 second, then catches and logs the error.

Execution Table

Step	Action	Promise State	Output
1	fetchData() called, Promise created	Pending
2	setTimeout triggers after 1s, reject called	Rejected
3	Promise catch runs due to rejection	Rejected	Error: Network error
4	No then block runs because Promise rejected	Rejected
5	Execution ends after catch handles error	Rejected	Error logged

💡 Promise rejected, catch block handled the error, stopping further then execution.

Variable Tracker

Variable	Start	After 1s	Final
Promise State	Pending	Rejected	Rejected
Output			Error: Network error

Key Moments - 2 Insights

Why doesn't the then block run when the Promise is rejected?

Because the Promise state changes to rejected (see step 2 and 3 in execution_table), only the catch block runs to handle the error.

What happens if we don't add a catch block?

Without catch, the rejected Promise causes an unhandled rejection error, which can crash the program or show warnings (not shown in this trace).

Visual Quiz - 3 Questions

Look at the execution_table, what is the Promise state at step 2?

A Rejected

B Resolved

C Pending

D Settled

💡 Hint

Check the 'Promise State' column at step 2 in execution_table.

At which step does the catch block run?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look for 'Promise catch runs' in the Action column of execution_table.

If the Promise resolved successfully instead of rejecting, what would change in the execution_table?

A Step 2 would show Rejected state

B Step 3 would show then block running instead of catch

C Step 5 would show error logged

D No change, catch still runs

💡 Hint

Refer to the Promise State and Output columns in execution_table for success vs error.

Concept Snapshot

Promise catch for async errors:
- Create a Promise for async work
- If error occurs, reject the Promise
- Use .catch() to handle errors
- .then() runs only if Promise resolves
- Catch prevents unhandled rejections
- Always add catch for safe async code

Full Transcript

This visual trace shows how a Promise handles asynchronous errors using catch. First, the Promise is created and starts pending. After 1 second, the Promise rejects with an error. Because of rejection, the catch block runs to handle the error, logging it. The then block does not run because the Promise did not resolve successfully. This pattern helps safely manage errors in async code by catching them and preventing crashes or warnings. Always add a catch block after then to handle possible errors.