Why is Nesting `async/await` within Promise Constructors an Anti-Pattern?

The Anti-Pattern of Nesting async/await within Promise Constructors

In this instance, where the async.eachLimit function is employed to manage the number of concurrent operations, a dilemma arises. The initial temptation is to structure the code as follows:

function myFunction() {
  return new Promise(async (resolve, reject) => {
    eachLimit((await getAsyncArray), 500, (item, callback) => {
      // Operations using native promises
    }, (error) => {
      if (error) return reject(error);
      // Resolve with the next value
    });
  });
}

While it may seem logical to declare the "myFunction" function as asynchronous, it is not feasible because the inner callback of the "eachLimit" function remains inaccessible. However, this approach poses a significant pitfall: the potential for errors to remain unhandled.

This approach is a textbook example of the anti-pattern that involves using promises within the constructor of another promise. In this case, the risk of unhandled errors is particularly acute. To avoid this, consider the following code snippet:

let p = new Promise(resolve => {
  ""(); // TypeError
  resolve();
});

(async () => {
  await p;
})().catch(e => console.log("Caught: " + e)); // Catches the error.

In this scenario, while the first line throws an exception, the error is gracefully handled by the "catch" block of the asynchronous arrow function. Proper error handling is critical to maintain stability and prevent unexpected behavior in your codebase.

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