46. What is callback in callback?

A callback in a callback in JavaScript refers to the practice of passing a callback function as an argument to another callback function. This pattern is common when handling asynchronous operations, where you need to perform a sequence of tasks after each step is completed.

Example:

Here's an example where one callback is passed into another callback:

javascript
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function firstTask(callback) {
  setTimeout(() => {
    console.log('First task completed');
    callback(); // Invoke the callback after the first task is done
  }, 1000);
}

function secondTask(callback) {
  setTimeout(() => {
    console.log('Second task completed');
    callback(); // Invoke the callback after the second task is done
  }, 1000);
}

function thirdTask() {
  console.log('Third task completed');
}

// Calling callbacks within callbacks
firstTask(() => {
  secondTask(() => {
    thirdTask();
  });
});

Explanation:

1. firstTask is executed, and after 1 second, it calls the callback.
2. The callback passed to firstTask is secondTask, which executes after the first task finishes.
3. secondTask calls the callback after another 1 second, which then calls thirdTask.
4. thirdTask finally executes once secondTask completes.

Why Use Callbacks in Callbacks?

• To handle asynchronous execution.
• To ensure that one task starts only after the previous one completes.
• To create a sequence of dependent operations.

Potential Issue:

• Nesting multiple callbacks like this can lead to callback hell (or the "Pyramid of Doom") due to increased complexity and indentation.
• Solution: Use Promises or async/await to simplify such patterns.