Promise.race and Promise.allSettled in Node.js - Step-by-Step Execution

Concept Flow - Promise.race and Promise.allSettled

Start multiple promises

↓

First promise settles

↓

Resolve or reject with first settled promise

↓

Wait for all promises to settle

↓

Return array of all settled results

Promise.race returns the result of the first promise that settles (resolve or reject). Promise.allSettled waits for all promises to finish and returns their results regardless of success or failure.

Execution Sample

Node.js

const p1 = new Promise(r => setTimeout(() => r('A'), 300));
const p2 = new Promise((_, rej) => setTimeout(() => rej('B'), 200));

Promise.race([p1, p2]).then(console.log).catch(console.error);

Promise.allSettled([p1, p2]).then(console.log);

Runs two promises: one resolves after 300ms, one rejects after 200ms. Shows how Promise.race returns first settled, Promise.allSettled waits for both.

Execution Table

Step	Promise	Action	Time (ms)	Result	Promise.race Outcome	Promise.allSettled Outcome
1	p1	Started	0	Pending	Pending	Pending
2	p2	Started	0	Pending	Pending	Pending
3	p2	Rejects	200	Rejected with 'B'	Rejected with 'B'	Pending
4	p1	Resolves	300	Resolved with 'A'	Rejected with 'B'	Pending
5	All settled	All promises settled	300	p1: fulfilled 'A', p2: rejected 'B'	Rejected with 'B'	[{status:'fulfilled', value:'A'}, {status:'rejected', reason:'B'}]

💡 Promise.race settles at 200ms with p2 rejection; Promise.allSettled waits until 300ms when both promises have settled.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
p1	Pending	Pending	Pending	Pending	Resolved 'A'
p2	Pending	Pending	Rejected 'B'	Rejected 'B'	Rejected 'B'
Promise.race Outcome	Pending	Pending	Rejected 'B'	Rejected 'B'	Rejected 'B'
Promise.allSettled Outcome	Pending	Pending	Pending	Pending	[{status:'fulfilled', value:'A'}, {status:'rejected', reason:'B'}]

Key Moments - 3 Insights

Why does Promise.race settle with the rejected promise even though another promise resolves later?

Because Promise.race settles as soon as the first promise settles, whether it resolves or rejects. See execution_table row 3 where p2 rejects first, so race settles immediately.

Why does Promise.allSettled wait for both promises even if one rejects early?

Promise.allSettled waits for all promises to settle regardless of resolve or reject. It collects all results. See execution_table rows 3 and 4 where it waits until both p1 and p2 finish.

What is the difference in the output format between Promise.race and Promise.allSettled?

Promise.race returns the value or reason of the first settled promise directly. Promise.allSettled returns an array of objects describing each promise's status and value or reason. See execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution_table at step 3: what is the Promise.race outcome?

A Rejected with 'B'

B Resolved with 'A'

C Pending

D Resolved with 'B'

💡 Hint

Check the 'Promise.race Outcome' column at step 3 in the execution_table.

At which step does Promise.allSettled return its final array of results?

A Step 3

B Step 5

C Step 4

D Step 2

💡 Hint

Look at the 'Promise.allSettled Outcome' column and find when it shows the full array.

If p2 resolved instead of rejected at 200ms, what would Promise.race return at step 3?

A Rejected with 'B'

B Resolved with 'A'

C Resolved with p2's value

D Pending

💡 Hint

Promise.race returns the first promise that settles, so if p2 resolves first, it returns p2's value.

Concept Snapshot

Promise.race(promises): returns the first promise that settles (resolve or reject).
Promise.allSettled(promises): waits for all promises to settle, returns array of {status, value/reason}.
Use race to get fastest result; use allSettled to get all results regardless of success.
Both return promises themselves.
Race settles early; allSettled waits for all.

Full Transcript

This visual trace shows how Promise.race and Promise.allSettled behave with two promises: one resolving after 300ms and one rejecting after 200ms. Promise.race settles as soon as the first promise settles, which is the rejection at 200ms, so it returns that rejection immediately. Promise.allSettled waits for both promises to finish, then returns an array describing each promise's outcome. Variables track promise states and outcomes step-by-step. Key moments clarify why race settles early and allSettled waits for all. The quiz tests understanding of timing and results based on the execution table.