The underlying HTTP(s) request is an asynchronous operation and does not block the execution of the rest of the JavaScript code. When you pass a callback function into another function, you just pass the reference of the function i.e., the function name without the parentheses (). How to Write a Callback Function It may be defined elsewhere and this function can be used later as a callback. Like so many elements of modern programming, the closure has its origins in the early Lisps. In this example, the isOddNumber is a callback function. Here, pi() is a callback function. The parentheses are not used when passing the callback function. Save the file with name callback.html and open it in any browser (Chrome, Firefox, or IE).It should show the output as: In the above example, we have passed the second() function as a callback function to first() function, and it ensures that the second() function invokes after all the processing of the first function has completed its execution only. 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 … Try to understand the following code. The callback function … 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. Anonymous functions are functions without a name. The anonymous function argument passed into the above example’s setTimeout function call is also a callback! 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. You can read more about jQuery’s callback functions here. If needed we can pass the name of an anonymous function. We are seeing that the call function is being called after execution of the a() function. Also, if you are already familiar with setTimeout, then you’ve been using callback functions all along. Anonymous Callback Functions When the fadeIn() method is completed, then the callback function (if present) will be executed. They usually appear inside other functions… This is an anonymous JavaScript function which contains a callback. In this example, we have passed the function name. This is a very basic example of callback functions. 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. Anonymous functions are used heavily in JavaScript for many things, most notably the many callbacks used by the language’s many frameworks. Example: So the example’s original callback is actually executed by another callback. To make it shorter, you can use an anonymous function as a callback: 0:02 An anonymous function is a function without a name. Non anonymous callback function: A callback function is not always required to be defined as an anonymous 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. 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 and closures are a useful concepts included in most modern programming languages. 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