Callbacks (Why They Still Matter)

What Is a Callback?

A callback is a function passed into another function to be executed later, usually after an asynchronous operation completes.

Classic Node Callback Pattern

fs.readFile('file.txt', (err, data) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(data.toString());
});

Error-First Convention

Node adopted the “error-first callback” convention: the first parameter is always an error object.

Callback Hell

step1((err, result1) => {
  step2(result1, (err, result2) => {
    step3(result2, (err, result3) => {
      console.log(result3);
    });
  });
});

Problems

• Deep nesting
• Hard error propagation
• Reduced readability

Production Insight

Callbacks still exist in low-level APIs, but modern backend code prefers promises and async/await.