Callbacks & Callback Hell

What pattern does this code implement?

const operations = [
  callback => setTimeout(() => callback(null, 1), 1000),
  callback => setTimeout(() => callback(null, 2), 500),
  callback => setTimeout(() => callback(null, 3), 800)
];

function parallel(ops, finalCallback) {
  const results = [];
  let completed = 0;
  
  ops.forEach((op, index) => {
    op((err, result) => {
      if (err) return finalCallback(err);
      results[index] = result;
      completed++;
      if (completed === ops.length) {
        finalCallback(null, results);
      }
    });
  });
}

Sequential execution
Parallel execution with callback
This code implements parallel execution of asynchronous operations with a callback. Key features: 1) Multiple operations run concurrently rather than sequentially, 2) Results are collected in order regardless of completion time, 3) Final callback is called only when all operations complete, 4) Error handling stops further processing, 5) Uses a counter to track completion status. This pattern is useful for improving performance when multiple independent async operations need to be executed.
Event loop
Error handling

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