We declare another function called pi() that returns the value of π. function pi() { return Math.PI; } and pass it as an argument to irrational() irrational(pi); The function irrational() executes it and prints the value 3.141592653589793 in the console. To make it shorter, you can use an anonymous function as a callback: Here, pi() is a callback function. So the example’s original callback is actually executed by another callback. As JavaScript has anonymous functions available, you can also pass in anonymous Functions as a variable to a function. It may be defined elsewhere and this function can be used later as a callback. Also, if you are already familiar with setTimeout, then you’ve been using callback functions all along. Callbacks are a great way to preserve a certain order of function executions, which means there is a chain of callbacks, the function A need executing first then B, then C…A callback function is just like other functions, it can be written as an anonymous function, and using the arrow function syntax. 0:02 An anonymous function is a function without a name. How to Write a Callback Function So, depending on the speed chosen, there could be a noticeable delay before the callback function code is executed. Another common way you'll encounter and 0:00 write callback functions is as anonymous functions. When the fadeIn() method is completed, then the callback function (if present) will be executed. You can read more about jQuery’s callback functions here. This is a very basic example of callback functions. We are seeing that the call function is being called after execution of the a() function. Like so many elements of modern programming, the closure has its origins in the early Lisps. If needed we can pass the name of an anonymous function. The underlying HTTP(s) request is an asynchronous operation and does not block the execution of the rest of the JavaScript code. 0:05 In the case of a callback an anonymous function can be written in place or 0:09 in line where the name of the function would be. function functionTwo(var1, callback) { callback(var1); } functionTwo(1, function (x) { alert(x); }) In this case, the callback function is defined when we are calling functionTwo. Anonymous functions are used heavily in JavaScript for many things, most notably the many callbacks used by the language’s many frameworks. Try to understand the following code. Anonymous functions and closures are a useful concepts included in most modern programming languages. In JavaScript, everything can be treated as an object, this means that a function can be sent into another function as a parameter and can also be retrieved from the called function … Example: The callback function … The parentheses are not used when passing the callback function. This is an anonymous JavaScript function which contains a callback. In this example, we have passed the function name. Anonymous Callback Functions Non anonymous callback function: A callback function is not always required to be defined as an anonymous function. In this example, the isOddNumber is a callback function. 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