Promise.all() vs. Multiple await: When Do Timing Differences Matter?

Await Promise.all() vs. Multiple Await: Timing Differences

In JavaScript, asynchronous operations can be handled using Promise.all() or multiple await statements. While both methods ultimately serve the same purpose of waiting for multiple promises to settle, there are some subtle timing differences between them.

Scenario 1: Using Promise.all()

Promise.all() takes an array of promises and returns a single promise that resolves to an array of results once all the input promises have settled, regardless of whether they resolve or reject.

Example:

const data = await Promise.all([res(3000), res(2000), res(1000)])

In this example, the Promise.all() method delays the resolution of data until all three input promises have resolved.

Scenario 2: Using Multiple await Statements

In this scenario, multiple await statements are used to wait for each individual promise to settle:

const t1 = task1();
const t2 = task2();

const result1 = await t1;
const result2 = await t2;

Here, result1 will be resolved as soon as task1() settles, and result2 will be resolved as soon as task2() settles.

Timing Comparison

The main timing difference between these two approaches is that Promise.all() delays the resolution of the final result until all input promises have settled. This can be advantageous in scenarios where it's important to wait for all tasks to complete before proceeding.

On the other hand, using multiple await statements allows for individual tasks to settle independently, potentially resulting in a faster overall completion time.

Example Illustration

Let's consider an example where delay functions are used to simulate asynchronous tasks:

Example #1 (Using Promise.all()):

const data = await Promise.all([res(3000), res(2000), res(1000)])

Example #2 (Using Multiple await Statements):

const t1 = task1();
const t2 = task2();
const t3 = task3();

const result1 = await t1;
const result2 = await t2;
const result3 = await t3;

In Example #1, the Promise.all() method would delay the resolution of data until all three tasks have completed, taking 3 seconds. In Example #2, since the tasks settle independently, result1 would be available after 1 second, result2 after 2 seconds, and result3 after 3 seconds.

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