How Can Promise.all Improve Concurrent Async Operation Handling and Avoid Unhandled Rejections?

Waiting for Concurrent Async Operations

Parallel Execution with Promise.all

To trigger multiple async operations concurrently, avoid using the pattern where you acquire promises and then await them sequentially. Instead, leverage the Promise.all function:

const [value1, value2] = await Promise.all([getValue1Async(), getValue2Async()]);

This approach initiates both async operations simultaneously and provides the results in an array.

Handling Rejected Promises

The provided solution runs the operations in parallel, but waits for the first to complete before waiting for the second. While this allows parallel execution, it introduces issues in handling rejected promises.

If the first promise takes longer to complete and the second fails, Promise.all will fail immediately, preventing the first promise from rejecting. This can result in an unhandled rejection error.

Syntax for Parallel Await

Currently, there isn't a designated await syntax for parallel waiting, hence the alternative of using Promise.all. Discussions have emerged regarding await.all, indicating the possibility of future enhancements.

Comparison of Approaches

Consider the following example, where getValue1Async takes 500ms to resolve and getValue2Async takes 100ms to reject:

Sequential Execution:

async () => {
  try {
    const value1 = await getValue1Async();
    const value2 = await getValue2Async();
  } catch (e) {
    console.error(e);
  }
};

Concurrent Execution with Promise.all:

async () => {
  try {
    const [value1, value2] = await Promise.all([getValue1Async(), getValue2Async()]);
  } catch (e) {
    console.timeEnd("Promise.all", e);
  }
};

Output:

• Sequential: Console output shows a delay of 500ms before the error is logged.
• Concurrent: Console output shows an error log after 100ms.

Conclusion:

Using Promise.all effectively enables parallel execution of async operations and ensures proper handling of rejected promises.

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