If you read this far, tweet to the author to show them you care. Tweet a thanks. Learn to code for free. Get started. Forum Donate. Cem Eygi. What is a Callback Function? In JavaScript, functions are objects. Can we pass objects to functions as parameters?
You can also watch the video version of callback functions below: Why do we need Callback Functions? The problem with the second example, is that you cannot prevent the calculator function from displaying the result. Using a callback, you could call the calculator function myCalculator with a callback, and let the calculator function run the callback after the calculation is finished:. Where callbacks really shine are in asynchronous functions, where one function has to wait for another function like waiting for a file to load.
We just launched W3Schools videos. Get certified by completing a course today! This function will make a web request using the fetch API. Our callback function prints the data returned from this request to the console:. Callbacks allow you to pass a function as a parameter inside another function. They are commonly used within event handlers to run code when an event is executed or to make web requests.
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In fact, even commonly used asynchronous functions like setTimeout are provided by libraries and interfaces. Access to the DOM and functions for manipulating it are built in to the language. As the use of JavaScript expanded into more areas, such as server-side functionality with Node. There is a companion repository for this post available on GitHub. Begin your exploration of concurrency in JavaScript by initializing a new Node.
You can initialize the project and its Git repository with a series of command line instructions. Open a console window and execute the following instructions in the directory where you want to create the project directory:. You can learn more about initializing Node. You can see how the JavaScript event loop works by writing a few simple functions.
In the first example the code executes synchronously from the stack and behaves the way you'd expect it to from looking at the code.
Then you'll write a function that behaves in a way you might not expect. Create a greet. Run the program with Node. You may know the setTimeout method. It accepts two parameters: the first is a function you want to execute and the second is the time, in milliseconds, the program should wait before executing the function. The function to be executed is passed to setTimeout as an argument. This is a callback function: it's a function you want to run after the function to which you're passing it as an argument does whatever it's supposed to do.
In the case of setTimeout , this just means waiting a number of milliseconds. Modify the greet function in the greet. You were probably expecting exactly the same output as the original greet. The only difference is that you used setTimeout to delay the greet name function's console output for one second.
So why does the output look reversed? Could it be the 1-second timeout? This is correct functionality, and it works this way because setTimeout causes the callback function to be executed after a given amount of time—but it does not block execution of the rest of the code on the stack. This is referred to as a "non-blocking" event model, which is one of JavaScript's distinguishing features: it never blocks code on the stack from executing. There are some exceptions to the single-threaded aspect of JavaScript, but they're beyond the scope of this post.
The output appears in this unexpected order because of the order of execution in JavaScript's event loop model. Each function in the stack executes completely before execution moves to the next function. You could think about function completion as the moment when it reaches the return statement. In the greet. When program execution begins the main function is the only function in the stack.
As each inner function is called it is pushed to the top of the stack, so its execution completes before execution resumes with the parent function. This can occur with many levels of nesting, which can make a program difficult to debug.
Here is where the JavaScript callback queue and event loop act as a mechanism to communicate with external APIs. Do your job, and when you are done leave me a callback function to be executed in the callback queue. This is why the messages appear out of order. The functions on the stack, including the one that displays "Nice to meet you", finish executing before functions in the callback queue are executed.
The following illustration shows the event loop described in the sequence of steps above.
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