Await Promise.all() vs. Multiple Await: When Should You Use Which?

Unveiling the Differences: Await Promise.all() vs. Multiple Await

In asynchronous JavaScript coding, developers face the dilemma of how to handle multiple concurrently executed tasks and wait for their results. Two common approaches emerge: await Promise.all() and multiple await. While both achieve the same goal, they exhibit subtle timing differences that can impact code execution.

Await Promise.all()

When utilizing await Promise.all([task1(), task2()]), the program suspends execution until all promises within the array are resolved. Essentially, it waits for the longest-running task to complete before moving on.

Multiple Await

In contrast, the separate await statements sequentially wait for each task:

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

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

Each await suspends execution until the corresponding task concludes.

Timing Differences

The crucial distinction lies in the timing of task execution. With Promise.all(), all tasks commence simultaneously, while with multiple await, tasks are initiated sequentially.

Consider this example:

// Delay 1 takes 500ms, Delay 2 takes 300ms
const [delay1, delay2] = [res(500), res(300)];

const t1 = await Promise.all([delay1, delay2]);

const [m1, m2] = await Promise.all([res(100), res(200)]);

In the first case, both delays start immediately, and Promise.all() concludes after 500ms, as Delay 1 is the longest-running task. In the second case, res(100) executes before res(200), potentially leading to a time advantage.

Conclusion

Both await Promise.all() and multiple await offer distinct approaches for handling asynchronous tasks. Understanding their timing differences is essential to optimize code execution and ensure efficient program performance.

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