Coding Challenge Practice - Question 91

Source: Dev.to

Problem Description

Implement the async function helper parallel().

The parallel function should accept an array of async functions, each having the signature func(callback). It must run all functions concurrently, collect their results in the original order, and invoke a final callback exactly once.

Requirements

• If the input array is empty, invoke the callback immediately with undefined error and an empty results array.
• Store each function’s result at the index corresponding to its position in the input array.
• Count completed tasks and call the final callback only after all tasks finish.
• If any function reports an error, invoke the callback with that error and ignore any further results.
• Preserve result order even when tasks finish out of order.

Implementation

function parallel(funcs) {
  // Returns a function that accepts the final callback
  return function (callback) {
    // Handle empty array case
    if (funcs.length === 0) {
      callback(undefined, []);
      return;
    }

    const results = [];
    let completed = 0;
    let hasError = false;

    // Run all functions in parallel
    funcs.forEach((func, index) => {
      func((error, result) => {
        // Prevent multiple callbacks after an error has occurred
        if (hasError) return;

        // Handle error from any function
        if (error) {
          hasError = true;
          callback(error, undefined);
          return;
        }

        // Store result preserving original order
        results[index] = result;
        completed++;

        // When all functions have completed, invoke the final callback
        if (completed === funcs.length) {
          callback(undefined, results);
        }
      });
    });
  };
}

Usage Example

// Example async functions
function asyncTask1(cb) {
  setTimeout(() => cb(undefined, 'result1'), 300);
}
function asyncTask2(cb) {
  setTimeout(() => cb(undefined, 'result2'), 100);
}
function asyncTaskWithError(cb) {
  setTimeout(() => cb(new Error('Oops'), undefined), 200);
}

// Run tasks in parallel
const runParallel = parallel([asyncTask1, asyncTask2]);

runParallel((err, results) => {
  if (err) {
    console.error('Error:', err);
  } else {
    console.log('Results:', results); // => ['result1', 'result2']
  }
});