Callback

What is a Callback?

In programming, a callback is a function passed as an argument to another function, which then invokes the callback at a later time. This allows one piece of code to supply custom behavior to another.

The receiving function “calls back” the supplied function when an event occurs or an operation completes.

For example, in C, a callback is passed via a function pointer and executed inside the receiving function.

How Callbacks Work

Callbacks rely on two key concepts: higher-order functions and function pointers. A higher-order function is simply a function that accepts another function as a parameter and later calls it.

• In dynamic languages like JavaScript, any function can be passed as an argument. The recipient executes it when appropriate. For instance, a greet() function can accept a callback function, print a message, and then execute the callback.
• In C, callbacks use function pointers—variables that store a function’s memory address. This pointer can then be passed into another function and invoked later.

Callbacks can be synchronous or asynchronous. A synchronous callback runs immediately after being passed. Asynchronous callbacks, however, execute later, often after waiting for an external event.

For example, JavaScript’s setTimeout() function accepts a callback that runs after a specified delay.

Asynchronous callbacks are fundamental to non-blocking programming, allowing code to continue executing while a long-running task (like fetching data from a server) completes.

Why Callbacks Are Important

Callbacks are a fundamental tool in programming for several reasons:

• Asynchronous Operations: In JavaScript, callbacks are used to handle tasks such as API requests, file system operations, and database queries. The callback function is executed only after the asynchronous operation has finished.
• Event-Driven Programming: User interface frameworks and web browsers rely on callbacks to respond to events like mouse clicks, keyboard input, and network events.
• Customizable Behavior: Functions can accept callbacks to change their behavior without altering their internal code. For example, a generic sorting function in C uses a callback to define custom comparison logic for different data types.
• Modularity and Reusability: Callbacks allow developers to write generic functions and supply specific functionality via parameters, making code more modular and reusable.

Callback Examples

Here are some typical callback scenarios:

• Basic Greeting in JavaScript: A greet() function prints a greeting and then executes a callback function passed to it. This is a simple demonstration of passing and invoking a callback.
• Delayed Execution: Using JavaScript’s setTimeout(), you can pass an anonymous function that runs after a specified delay, allowing the rest of the program to continue executing in the meantime.
• Custom Calculator in C: A calc() function can accept two numbers and a callback function representing an operation (add, subtract, multiply, divide). The calc() function then executes the callback to compute the result.
• Event Listeners: In web development, callbacks are used to handle events like button clicks. The anonymous function defined within an event listener is the callback.
• Sorting with Custom Comparison: In C’s qsort() or similar functions, a comparison callback determines the ordering of elements. Passing different callbacks allows the same sorting function to sort by different criteria.

The Callback Process (Step by Step)

In languages like C, the process of using a callback follows a specific sequence:

1. Define the Callback: Write the function that will be called later (e.g., a comparison or logging function).
2. Declare a Function Pointer: Declare a variable that can hold a pointer to the callback, ensuring it has the same function signature.
3. Pass the Pointer: Call a higher-order function and pass the function pointer as an argument.
4. Invoke the Callback: Inside the higher-order function, the callback is called using the function pointer.

In JavaScript, this process is simpler: you simply define a function and pass it as an argument; the receiving function then calls it when needed.

Related Concepts

Understanding callbacks opens the door to several related topics:

• Higher-Order Functions: A broader category of functions that take other functions as arguments or return functions.
• Closures: Functions that retain access to variables from their lexical scope, often used effectively with callbacks.
• Promises and async/await: Modern constructs for handling asynchronous operations with improved readability and error handling.
• Event Loop: The mechanism that schedules and executes callbacks in asynchronous environments like JavaScript.

Conclusion

A callback is a function passed to another function and executed later. It enables one piece of code to respond to events, asynchronous operations, or customizable behavior. Callbacks are widely used in event-driven and asynchronous programming, as well as in generic algorithms like sorting.

By mastering callbacks, computer science students gain insight into higher-order functions, function pointers, and asynchronous code patterns. While they offer flexibility and modularity, callbacks also introduce challenges like callback hell and complex error handling, which can be mitigated with modern alternatives like Promises and async/await