Creating promises in Javascript - Visual Walkthrough

Concept Flow - Creating promises

Start

↓

Create new Promise

↓

Executor function runs

↓

Call resolve(value)

↓

Promise fulfilled

↓

Call reject(error)

↓

Promise rejected

↓

Promise settles (fulfilled or rejected)

↓

Then or catch handlers run

↓

End

This flow shows how a new Promise is created, the executor function runs, and either resolve or reject is called to settle the promise, triggering then or catch handlers.

Execution Sample

Javascript

const p = new Promise((resolve, reject) => {
  setTimeout(() => resolve('Done'), 1000);
});
p.then(result => console.log(result));

Creates a promise that resolves with 'Done' after 1 second, then logs the result.

Execution Table

Step	Action	Executor Function	Promise State	Output
1	Create Promise	Executor function starts	pending	none
2	setTimeout scheduled	Waiting 1 second	pending	none
3	Timeout fires	Calls resolve('Done')	fulfilled	none
4	Then handler runs	Receives 'Done'	fulfilled	Logs: Done
5	End	Promise settled	fulfilled	none

💡 Promise settles after resolve is called, then handler runs and logs output.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
p	Promise pending	Promise pending	Promise pending	Promise fulfilled	Promise fulfilled
result	undefined	undefined	undefined	undefined	'Done'

Key Moments - 3 Insights

Why does the promise start in 'pending' state and not immediately 'fulfilled'?

Because the executor function runs immediately but the asynchronous operation inside (setTimeout) delays calling resolve, so the promise stays 'pending' until resolve is called (see execution_table step 2 and 3).

What happens if reject is called instead of resolve?

The promise state changes to 'rejected' and the catch handler would run instead of then (not shown here, but similar flow to step 3 and 4).

Why does the then handler run after the promise is fulfilled?

Because then handlers wait for the promise to settle (fulfilled or rejected) before running, as shown in execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the promise state at step 2?

A rejected

B fulfilled

C pending

D settled

💡 Hint

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

At which step does the then handler run and log the result?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for 'Then handler runs' and 'Logs: Done' in the Output column of execution_table.

If the executor called reject instead of resolve, what would change in the execution table?

A Promise state would be 'rejected' at step 3

B Then handler would run at step 4

C Promise state would be 'fulfilled' at step 3

D No change in promise state

💡 Hint

Recall that calling reject changes promise state to 'rejected' instead of 'fulfilled' (see key_moments).

Concept Snapshot

Creating Promises in JavaScript:
- Use new Promise((resolve, reject) => {...})
- Executor runs immediately
- Call resolve(value) to fulfill
- Call reject(error) to reject
- Then/catch handle results asynchronously
- Promise starts as 'pending' and settles later

Full Transcript

This visual trace shows how creating a promise works in JavaScript. First, a new Promise is created and the executor function runs immediately. Inside, asynchronous work like setTimeout is scheduled. The promise starts in a 'pending' state. When resolve is called after 1 second, the promise state changes to 'fulfilled'. Then, the then handler runs and logs the result 'Done'. This flow helps understand how promises manage asynchronous operations by settling later and triggering handlers.