Promise error handling in Javascript - Step-by-Step Execution

Concept Flow - Promise error handling

Create Promise

↓

Promise runs async task

↓

Task succeeds?

No→Reject with error

↓

Catch error handler

↓

Resolve with value

↓

Handle error

↓

Then handler runs

↓

End

This flow shows how a Promise runs an async task, then either resolves successfully or rejects with an error, which can be caught and handled.

Execution Sample

Javascript

const p = new Promise((resolve, reject) => {
  reject('Error happened');
});

p.then(value => console.log('Success:', value))
 .catch(err => console.log('Caught:', err));

This code creates a Promise that immediately rejects with an error, then catches and logs that error.

Execution Table

Step	Action	Promise State	Handler Called	Output
1	Create Promise	pending	none
2	Promise executor calls reject('Error happened')	rejected	none
3	Then handler skipped (promise rejected)	rejected	none
4	Catch handler called with error	rejected	catch	Caught: Error happened
5	End of promise chain	settled	none

💡 Promise is rejected, catch handler handles the error, chain ends.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
p (Promise)	pending	rejected	rejected	settled
error	undefined	'Error happened'	'Error happened'	'Error happened'

Key Moments - 2 Insights

Why does the then handler not run when the promise is rejected?

Because in the execution_table at step 3, the promise state is rejected, so the then handler is skipped and only the catch handler runs.

How does the catch handler receive the error?

At step 4 in the execution_table, the catch handler is called with the rejection reason 'Error happened' passed as the error argument.

Visual Quiz - 3 Questions

Look at the execution_table, what is the promise state after step 2?

A rejected

B resolved

C pending

D settled

💡 Hint

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

At which step does the catch handler run?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Handler Called' and 'Output' columns in the execution_table.

If the promise resolved instead of rejected, which handler would run?

A catch handler

B then handler

C both handlers

D no handler

💡 Hint

Refer to the concept_flow where success leads to then handler running.

Concept Snapshot

Promise error handling:
- Create a Promise with resolve and reject
- If async task fails, call reject(error)
- Use .catch() to handle errors
- .then() runs only on success
- Errors skip then and go to catch

Full Transcript

This visual trace shows how a JavaScript Promise handles errors. First, a Promise is created and starts pending. Then the executor calls reject with an error, changing the state to rejected. Because the Promise is rejected, the then handler is skipped and does not run. Instead, the catch handler runs and receives the error, logging it. Finally, the Promise chain ends. This shows that catch is used to handle errors from rejected Promises, while then only runs on success.